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<!DOCTYPE html>
<html lang="en">
<head>
<meta charset="utf-8">
<title>RDF 1.2 Concepts and Abstract Syntax</title>
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<section id="abstract">
<p>The Resource Description Framework (RDF) is a framework for
representing information in the Web.
This document defines an abstract syntax (a data model)
which serves to link all RDF-based languages and specifications.
The abstract syntax has two key data structures:</p>
<ul>
<li>RDF graphs are sets of subject-predicate-object triples,
where the elements may be IRIs, blank nodes, or datatyped literals.
They are used to express descriptions of resources.</li>
<li>RDF datasets are used to organize collections of RDF graphs,
are comprised of a default graph and zero or more named graphs.</li>
</ul>
<p>RDF 1.2 introduces the ability to use an <a>RDF triple</a>
as a <a>triple term</a>, in the <a>object</a> position of another <a>triple</a>.
RDF 1.2 also introduces <a>directional language-tagged strings</a>,
which contain a <a>base direction</a> element that allows the
initial text direction to be specified for presentation by a user agent.</p>
<p>RDF 1.2 Concepts introduces key concepts and terminology for RDF 1.2, discusses
datatyping, and the handling of <a>fragment identifiers</a> in IRIs within
RDF graphs.</p>
</section>
<section id="sotd" class="updateable-rec">
<p>This document is part of the RDF 1.2 document suite.
It is the central RDF 1.2 specification and defines the core RDF concepts.
Test suites and implementation reports of a number of RDF 1.2
specifications that build on this document are available through the
[[[RDF11-TESTCASES]]] document [[RDF11-TESTCASES]].</p>
<p>RDF 1.2 Concepts is an update to [[RDF11-CONCEPTS]],
which was itself, an update to [[RDF-CONCEPTS-20040210]].</p>
<p class="ednote">Determine how to reference 1.2 test cases.</p>
<section id="related" data-include="./common/related.html"></section>
</section>
<section id="section-Introduction" class="informative">
<h2>Introduction</h2>
<p>The <em>Resource Description Framework</em> (RDF) is a framework
for representing information in the Web.</p>
<p>This document defines an abstract syntax (a data model)
which serves to link all RDF-based languages and specifications,
including the following:</p>
<ul>
<li>the formal model-theoretic semantics for RDF [[RDF12-SEMANTICS]]</li>
<li>serialization syntaxes for storing and exchanging RDF such as [[[RDF12-TURTLE]]] [[RDF12-TURTLE]]
and [[[JSON-LD11]]] [[JSON-LD11]]</li>
<li>the [[[SPARQL12-QUERY]]] [[SPARQL12-QUERY]]</li>
<li>the [[[RDF12-SCHEMA]]] [[RDF12-SCHEMA]]</li>
</ul>
<section id="data-model">
<h3>Graph-based Data Model</h3>
<p>The core structure of the abstract syntax is a set of
<a data-lt="RDF triple">triples</a>, each consisting of a <a>subject</a>,
a <a>predicate</a> and an <a>object</a>. A set of such triples is called
an <a>RDF graph</a>. An RDF graph can be visualized as a node and
directed-arc diagram, in which each triple is represented as a
node-arc-node link.</p>
<figure id="fig-rdf-graph">
<a href="rdf-graph.svg"><img src="rdf-graph.svg" alt="An RDF graph with two nodes (Subject and Object) and a triple connecting them (Predicate)" /></a>
<figcaption>An RDF graph with two nodes (Subject and Object) and a triple connecting them (Predicate)</figcaption>
</figure>
<p>There can be four kinds of <a>nodes</a> in an
<a>RDF graph</a>: <a>IRIs</a>, <a>literals</a>,
<a>blank nodes</a>, and <a>triple terms</a>.</p>
<p class="issue" data-number="129">There is a mixture of "Abstract Syntax" and "Data Model". We should have a consistent way to say "Abstract Syntax" vs "Data Model". One way is to use "Abstract Syntax" as the basis of semantics and usually say "Data Model" in Concepts otherwise.</p>
</section>
<section id="resources-and-statements">
<h3>Resources and Statements</h3>
<p>Any <a>IRI</a> or <a>literal</a> <dfn data-lt="denote" data-local-lt="denoted">denotes</dfn>
something in the world (the "universe of discourse").
These things are called
<span id="dfn-resources"><!-- obsolete term --></span>
<dfn data-lt="resource">resources</dfn>. Anything can be a resource,
including physical things, documents, abstract concepts, numbers
and strings; the term is synonymous with "entity" as it is used in
[[[RDF12-SEMANTICS]]] [[RDF12-SEMANTICS]].
The resource denoted by an IRI is called its <a>referent</a>, and the
resource denoted by a literal is called its
<a>literal value</a>. Literals have
<a>datatypes</a> that define the range of possible
values, such as strings, numbers, and dates. Special kinds of literals —
<a>language-tagged strings</a> and <a>directional language-tagged strings</a> —
respectively denote plain-text strings in a natural language, and plain-text
strings in a natural language including an initial text direction.</p>
<p>Asserting an <a>RDF triple</a> says that <em>some relationship,
indicated by the <a>predicate</a>, holds between the
<a>resources</a> <a>denoted</a> by
the <a>subject</a> and <a>object</a></em>. This statement corresponding
to an RDF triple is known as an <dfn data-local-lt="statement">RDF statement</dfn>.
The predicate itself is an <a>IRI</a> and denotes a <dfn class="export">property</dfn>,
that is, a <a>resource</a> that can be thought of as a binary relation.
(Relations that involve more than two entities can only be
<a data-cite="SWBP-N-ARYRELATIONS#">indirectly
expressed in RDF</a> [[SWBP-N-ARYRELATIONS]].)</p>
<p>Unlike <a>IRIs</a> and <a>literals</a>,
<a>blank nodes</a> do not identify specific
<a>resources</a>. <a>Statements</a>
involving blank nodes say that something with the given relationships
exists, without explicitly naming it.</p>
</section>
<section id="referents">
<h3>The Referent of an IRI</h3>
<p>The <a>resource</a> <a>denoted</a> by an <a>IRI</a>
is also called its <dfn>referent</dfn>. For some IRIs with particular
meanings, such as those identifying XSD datatypes, the referent is
fixed by this specification. For all other IRIs, what exactly is
<a>denoted</a> by any given IRI is not defined by this specification. Other
specifications may fix IRI referents, or apply other constraints on
what may be the referent of any IRI.</p>
<p>Guidelines for determining the <a>referent</a> of an <a>IRI</a> are
provided in other documents, like
[[[WEBARCH]]] [[WEBARCH]]
and [[[COOLURIS]]] [[COOLURIS]].
A very brief, informal, and partial account follows:</p>
<ul>
<li>By design, IRIs have global scope. Thus, two different appearances of an IRI
<a>denote</a> the same <a>resource</a>. Violating this principle constitutes
an <a data-cite="WEBARCH#URI-collision">IRI collision</a> [[WEBARCH]].</li>
<li>By social convention, the
<a data-cite="WEBARCH#uri-ownership">IRI owner</a>
[[WEBARCH]] gets to say what the intended (or usual)
referent of an <a>IRI</a> is. Applications and users need not
abide by this intended denotation, but there may be a loss of
interoperability with other applications and users if they do
not do so.</li>
<li>The IRI owner can establish the intended <a>referent</a>
by means of a specification or other document that explains
what is denoted. For example, the
[[[VOCAB-ORG]]] [[VOCAB-ORG]]
specifies the intended referents of various IRIs that start with
<code>http://www.w3.org/ns/org#</code>.</li>
<li>A good way of communicating the intended referent
is to set up the IRI so that it
<a data-cite="WEBARCH#uri-dereference">dereferences</a> [[WEBARCH]]
to such a document.</li>
<li>Such a document can, in fact, be an <a>RDF document</a>
that describes the denoted resource by means of
<a>RDF statements</a>.</li>
</ul>
<p>Perhaps the most important characteristic of <a>IRIs</a>
in web architecture is that they can be
<a data-cite="WEBARCH#uri-dereference">dereferenced</a>,
and hence serve as starting points for interactions with a remote server.
This specification is not concerned with such interactions.
It does not define an interaction model. It only treats IRIs as globally
unique identifiers in a graph data model that describes resources.
However, those interactions are critical to the concept of
[[[LINKED-DATA]]], [[LINKED-DATA]],
which makes use of the RDF data model and serialization formats.</p>
</section>
<section id="vocabularies">
<h3>RDF Vocabularies and Namespace IRIs</h3>
<p>An <dfn data-local-lt="vocabulary">RDF vocabulary</dfn> is a collection of <a>IRIs</a>
intended for use in <a>RDF graphs</a>. For example,
the IRIs documented in [[RDF12-SCHEMA]] are the RDF Schema vocabulary.
RDF Schema can itself be used to define and document additional
RDF vocabularies. Some such vocabularies are mentioned in the
Primer [[RDF12-PRIMER]].</p>
<p>The <a>IRIs</a> in an <a>RDF vocabulary</a> often begin with
a common substring known as a <dfn>namespace IRI</dfn>.
Some namespace IRIs are associated by convention with a short name
known as a <dfn>namespace prefix</dfn>. Some examples:
<table id="tab-vocab-ns" class="simple">
<caption>Some example namespace prefixes and IRIs</caption>
<tr>
<th>Namespace prefix</th>
<th>Namespace IRI</th>
<th>RDF vocabulary</th>
</tr>
<tr>
<td>rdf</td>
<td><a href="http://www.w3.org/1999/02/22-rdf-syntax-ns#"><code>http://www.w3.org/1999/02/22-rdf-syntax-ns#</code></a></td>
<td>The RDF built-in vocabulary [[RDF12-SCHEMA]]</td>
</tr>
<tr><td>rdfs</td>
<td><a href="http://www.w3.org/2000/01/rdf-schema#"><code>http://www.w3.org/2000/01/rdf-schema#</code></a></td>
<td>The RDF Schema vocabulary [[RDF12-SCHEMA]]</td>
</tr>
<tr><td>xsd</td>
<td><a href="http://www.w3.org/2001/XMLSchema#"><code>http://www.w3.org/2001/XMLSchema#</code></a></td>
<td>The <a>RDF-compatible XSD types</a></td>
</tr>
</table>
<p>In some serialization formats it is common to abbreviate <a>IRIs</a>
that start with <a>namespace IRIs</a> by using a
<a>namespace prefix</a> in order to assist readability. For example, the IRI
<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#XMLLiteral</code>
would be abbreviated as <code>rdf:XMLLiteral</code>.
Note however that these abbreviations are <em>not</em> valid IRIs,
and must not be used in contexts where IRIs are expected.
Namespace IRIs and namespace prefixes are <em>not</em> a formal part of the
RDF data model. They are merely a syntactic convenience for
abbreviating IRIs.</p>
<p>The term “<dfn class="lint-ignore">namespace</dfn>” on its own does not have a
well-defined meaning in the context of RDF, but is sometimes informally
used to mean “<a>namespace IRI</a>” or “<a>RDF vocabulary</a>”.</p>
</section>
<section id="section-triple-terms-reification">
<h3>Triple Terms and Reification</h3>
<p>A <a>triple term</a> is an [=RDF triple=] used as an [=RDF term=]
in the [=object=] position of another triple.</p>
<p>A <a>triple term</a> is not necessarily asserted, allowing
statements to be made about other statements that may not be
asserted within an <a>RDF graph</a>.
This allows statements to be made about relationships that may be contradictory.
For a <a>triple term</a> to be asserted,
it must also appear in a graph as an <a>asserted triple</a>.</p>
<p>A <a>triple term</a> can be used as the object of a <a>triple</a> with the predicate <code>rdf:reifies</code>;
such a <a>triple</a> is then a <dfn>reifying triple</dfn>.
The <a>subject</a> of that <a>triple</a> is called a <dfn>reifier</dfn>.
Assertions on the <a>triple term</a> are made using the <a>reifier</a>.
A concrete syntax may provide a special notation for specifying <a>reifying triples</a>.</p>
<p class="note">Using the <a>reifier</a> as an indirect way of referencing a <a>triple term</a>
allows multiple groups of assertions to be separated from each other,
which might be necessary if the same <a>triple term</a> was derived from different sources.
Assertions are always made using the <a>reifier</a>, which can be the <a>subject</a> or <a>object</a>
of different triples.
Concrete syntaxes, such as Turtle [[RDF12-TURTLE]],
may have shortcuts for capturing a <a>triple term</a> with its <a>reifier</a>.</p>
<p>The following diagram represents a statement and a reification of an unasserted <a>triple term</a>.</p>
<figure id="fig-triple-term">
<a href="triple-term.svg">
<!-- A version of this graphic can be found at https://docs.google.com/drawings/d/1RP9Nw5GCNjduWSXZj_EZ8FaAc7p_N-WVmzJ0syvum1k -->
<img src="triple-term.svg"
alt="An RDF graph containing a triple that references an unasserted triple term (with grey background) via a reifier"
style="width:70%"
aria-describedby="fig-triple-term-alt"/>
</a>
<figcaption id="fig-triple-term-alt">
An <a>RDF graph</a> containing a <a>triple</a> that references an unasserted <a>triple term</a> (with grey background) via a <a>reifier</a>.
</figcaption>
</figure>
<p>A variation on the graph shown in <a href="#fig-triple-term"></a> can be described
where the <a>triple term</a> is also <a data-lt="asserted triple">asserted</a>.</p>
<figure id="fig-asserted-triple-term">
<a href="asserted-triple-term.svg">
<!-- A version of this graphic can be found at https://docs.google.com/drawings/d/1gAHQj4wk87tmeHf4-wnxdgmjq3T05_ovrJq0V-1VXKw -->
<img src="asserted-triple-term.svg"
alt="An RDF graph containing a triple that references an triple term, which is also asserted, via a reifier"
style="width:70%"
aria-describedby="fig-asserted-triple-term-alt"/>
</a>
<figcaption id="fig-asserted-triple-term-alt">
An <a>RDF graph</a> containing a <a>triple</a> that references an <a>triple term</a>, which is also asserted, via a <a>reifier</a>.
</figcaption>
</figure>
<p>Note that a <a>triple term</a> may also have another <a>triple term</a> as an <a>object</a>.</p>
</section>
<section id="change-over-time">
<h3>RDF and Change over Time</h3>
<p>The RDF data model is <em>atemporal</em>: <a>RDF graphs</a>
are static snapshots of information.</p>
<p>However, <a>RDF graphs</a> can express information
about events and about temporal aspects of other entities,
given appropriate <a>vocabulary</a> terms.</p>
<p>Since <a>RDF graphs</a> are defined as mathematical
sets, adding or removing <a>triples</a> from an
RDF graph yields a different RDF graph.</p>
<p>We informally use the term <dfn>RDF source</dfn> to refer to a
persistent yet mutable source or container of
<a>RDF graphs</a>. An RDF source is a <a>resource</a>
that may be said to have a state that can change over time.
A snapshot of the state can be expressed as an RDF graph.
For example, any web document that has an RDF-bearing representation
may be considered an RDF source. Like all resources, RDF sources may
be named with <a>IRIs</a> and therefore described in
other RDF graphs.</p>
<p>Intuitively speaking, changes in the universe of discourse
can be reflected in the following ways:</p>
<ul>
<li>An <a>IRI</a>, once minted, should never
change its intended <a>referent</a>. (See
<a data-cite="WEBARCH#URI-persistence">URI persistence</a>
[[WEBARCH]].)</li>
<li><a>Literals</a>, by design, are constants and
never change their <a data-lt="literal value">value</a>.</li>
<li>A relationship that holds between two <a>resources</a>
at one time may not hold at another time.</li>
<li><a>RDF sources</a> may change their state over time.
That is, they may provide different <a>RDF graphs</a>
at different times.</li>
<li>Some <a>RDF sources</a> may, however, be immutable
snapshots of another RDF source, archiving its state at some
point in time.</li>
</ul>
</section>
<section id="managing-graphs">
<h3>Working with Multiple RDF Graphs</h3>
<p>As RDF graphs are sets of triples, they can be
combined easily, supporting the use of data from
multiple sources. Nevertheless, it is sometimes desirable to work
with multiple RDF graphs while keeping their contents separate.
<a>RDF datasets</a> support this requirement.</p>
<p>An <a>RDF dataset</a> is a collection of
<a>RDF graphs</a>. All but one of these graphs have
an associated <a>IRI</a> or blank node. They are called
<a>named graphs</a>, and the IRI or blank node
is called the <a>graph name</a>.
The remaining graph does not have an associated IRI, and is called
the <a>default graph</a> of the RDF dataset.</p>
<p>There are many possible uses for <a>RDF datasets</a>.
One such use is to hold snapshots of multiple
<a>RDF sources</a>.</p>
</section>
<section id="entailment">
<h3>Equivalence, Entailment and Inconsistency</h3>
<p>An <a>RDF triple</a> encodes a <dfn class="export">proposition</dfn> — a
simple logical expression, describing a relationship between two entities.
An [=asserted triple=] is a claim that the corresponding proposition is true.
An <a>RDF graph</a> is the conjunction (logical <em>AND</em>) of
all the claims made by its [=asserted triples=].
The precise details of this meaning of [=RDF triples=] and [=RDF graphs=] are
the subject of [[[RDF12-SEMANTICS]]] [[RDF12-SEMANTICS]], which yields the
following relationships between <a>RDF graphs</a>:</p>
<dl>
<dt><dfn>Entailment</dfn></dt>
<dd>An <a>RDF graph</a> <em>A</em> entails another RDF graph <em>B</em>
if every possible arrangement of the world
that makes <em>A</em> true also makes <em>B</em> true. When <em>A</em>
entails <em>B</em>, if the truth of <em>A</em> is presumed or demonstrated
then the truth of <em>B</em> is established.
</dd>
<dt><dfn data-local-lt="equivalent">Equivalence</dfn></dt>
<dd>Two <a>RDF graphs</a> <em>A</em> and <em>B</em>
are equivalent if they make the same claim about the world.
<em>A</em> is equivalent to <em>B</em> if and only if
<em>A</em> <a data-lt="entailment">entails</a> <em>B</em> and
<em>B</em> entails <em>A</em>.
</dd>
<dt><dfn data-lt="inconsistent">Inconsistency</dfn><span id="dfn-inconsistency"><!-- obsolete term --></span></dt>
<dd>An <a>RDF graph</a> is inconsistent if it contains
an internal contradiction. There is no possible arrangement
of the world that would make the expression true.</dd>
</dl>
<p class="issue" data-number="118"></p>
<p>An <span id="dfn-entailment-regime"><!-- obsolete term --></span><dfn data-cite="RDF12-SEMANTICS#dfn-entailment-regime">entailment regime</dfn> [[RDF12-SEMANTICS]] is a specification that
defines precise conditions that make these relationships hold.
RDF itself recognizes only some basic cases of entailment, <a>equivalence</a>
and inconsistency. Other specifications, such as
[[[RDF12-SCHEMA]]] [[RDF12-SCHEMA]]
and <a data-cite="owl2-overview#">OWL 2</a>
[[OWL2-OVERVIEW]], add more powerful entailment regimes,
as do some domain-specific <a>vocabularies</a>.</p>
<p>This specification does not constrain how implementations
use the logical relationships defined by
<a>entailment regimes</a>.
Implementations may or may not detect
<a>inconsistencies</a>, and may make all,
some or no <a data-lt="entailment">entailed</a> information
available to users.</p>
</section>
<section id="rdf-documents">
<h3>RDF Documents and Syntaxes</h3>
<p>An <dfn>RDF document</dfn> is a document that encodes an
<a>RDF graph</a> or <a>RDF dataset</a> in a <dfn>concrete RDF syntax</dfn>,
such as Turtle [[RDF12-TURTLE]], RDFa [[RDFA-CORE]], JSON-LD [[JSON-LD11]], or
TriG [[RDF12-TRIG]]. RDF documents enable the exchange of RDF graphs and RDF
datasets between systems.</p>
<p>A <a>concrete RDF syntax</a> may offer
many different ways to encode the same <a>RDF graph</a> or
<a>RDF dataset</a>, for example through the use of
<a>namespace prefixes</a>,
<a>IRI references</a>, <a>blank node identifiers</a>,
and different ordering of triples. While these aspects can have great
effect on the convenience of working with the <a>RDF document</a>,
they are not significant for its meaning.</p>
</section>
</section>
<section id="conformance">
<p>This specification, <em>RDF 1.2 Concepts and Abstract Syntax</em>,
defines a data model and related terminology for use in
other specifications, such as
<a>concrete RDF syntaxes</a>,
API specifications, and query languages.
Implementations cannot directly conform to
<em>RDF 1.2 Concepts and Abstract Syntax</em>,
but can conform to such other specifications that normatively
reference terms defined here.</p>
<p>This specification establishes two conformance levels:</p>
<ul>
<li><dfn class="no-export lint-ignore" data-lt="full">Full conformance</dfn>
supports <a data-lt="RDF graph">graphs</a> and <a data-lt="RDF dataset">datasets</a>
with <a>triples</a> that contain <a>triple terms</a>.
Concrete syntaxes in which such graphs and datasets can be expressed include
[[RDF12-N-TRIPLES]], [[RDF12-N-QUADS]], [[RDF12-TURTLE]], and [[RDF12-TRIG]].</li>
<li><dfn class="no-export lint-ignore" data-lt="classic">Classic conformance</dfn>
only supports <a data-lt="RDF graph">graphs</a> or <a data-lt="RDF dataset">datasets</a>
with <a>triples</a> that do not contain <a>triple terms</a>.</li>
</ul>
<p class="ednote">The conformance levels described above are tentative,
and still the subject of group discussion. An alternative to conformance
levels, "profiles", may be adopted instead, abandoned, or described in
another specification.</p>
<p class="issue" data-number="70">
Change "Classic Conformance" to "Basic Conformance" and define them as profiles.</p>
<section id="rdf-strings">
<h3>Strings in RDF</h3>
<p>RDF uses Unicode [[Unicode]] as the fundamental representation for string values.
Within this, and related specifications, the term <dfn id="dfn-rdf-string">string</dfn>,
or <a data-lt="string">RDF string</a>,
is used to describe an ordered sequence of zero or more
<a data-cite="I18N-GLOSSARY#dfn-code-point" class="lint-ignore">Unicode code points</a>
which are <a data-cite="I18N-GLOSSARY#dfn-scalar-value" class="lint-ignore">Unicode scalar values</a>.
Unicode scalar values do not include the
<a data-cite="I18N-GLOSSARY#dfn-surrogate" class="lint-ignore">surrogate code points</a>.
Note that most <a>concrete RDF syntaxes</a> require the use
of the UTF-8 character encoding [[!RFC3629]],
and use the `\u0000` or `\U00000000` forms to express certain non-character values.
</p>
<p>A string is identical to another string if it consists of the same sequence of code points.
An implementation MAY determine string equality by comparing the
<a data-cite="I18N-GLOSSARY#dfn-code-unit">code units</a> of two strings
that use the same <a data-cite="I18N-GLOSSARY#dfn-character-encoding">Unicode character encoding</a>
(UTF-8 or UTF-16) without decoding the string into a
<a data-cite="I18N-GLOSSARY#dfn-code-point" class="lint-ignore">Unicode code point</a> sequence.</p>
</section>
</section>
<section id="section-rdf-graph">
<h2>RDF Graphs</h2>
<p>An <dfn>RDF graph</dfn> is a set of <a>RDF triples</a>.</p>
<p>An [=RDF triple=] is said to be <dfn id="dfn-asserted-triple" class=export data-lt="asserted triple|asserted triples" data-lt-no-plural=""
>asserted</dfn> in an [=RDF graph=] if it is an element of the [=RDF graph=].
</p>
<section id="section-triples">
<h3>Triples</h3>
<p>An <dfn data-local-lt="triple" class="export">RDF triple</dfn>
(often simply called "triple")
is a 3-tuple that is defined inductively as follows:</p>
<ul>
<li>
If |s| is an [=IRI=] or a [=blank node=],
|p| is an [=IRI=], and
|o| is an [=IRI=], a [=blank node=], or a [=literal=],
then (|s|, |p|, |o|) is an [=RDF triple=].
</li>
<li>
If |s| is an [=IRI=] or a [=blank node=],
|p| is an [=IRI=], and
|o| is an [=RDF triple=],
then (|s|, |p|, |o|) is an [=RDF triple=].
</li>
</ul>
<p>The three components (|s|, |p|, |o|) of an [=RDF triple=] are respectively called the <dfn class=export >subject</dfn>, <dfn class=export >predicate</dfn> and <dfn class=export >object</dfn> of the triple.</p>
<p class="note">The definition of <a>triple</a> is recursive.
That is, a <a>triple</a> can itself have an
<a>object</a> component which is another <a>triple</a>.
However, by this definition, cycles of <a>triples</a> cannot be created.</p>
</section>
<section id="section-terms">
<h3>RDF Terms</h3>
<p><a>IRIs</a>, <a>literals</a>,
<a>blank nodes</a>, and <a>triple terms</a> are collectively known as
<span id="dfn-rdf-terms"><!-- obsolete term--></span><dfn data-lt="rdf term">RDF terms</dfn>.</p>
<p><a>IRIs</a>, <a>literals</a>,
<a>blank nodes</a>, and <a>triple terms</a> are distinct and distinguishable.
For example, a literal with the string <code>http://example.org/</code> as
its <a>lexical form</a>
is not equal to the IRI <code>http://example.org/</code>,
nor to a blank node with the <a>blank node identifier</a>
<code>http://example.org/</code>.</p>
<p>The set of <span id="dfn-nodes"><!-- obsolete term--></span><dfn data-lt="node">nodes</dfn> of an <a>RDF graph</a>
is the set of <a>subjects</a> and <a>objects</a> of the <a>asserted triples</a> of the graph.
It is possible for a [=predicate=] [=IRI=] to also occur as a [=node=] in
the same graph.</p>
</section>
<section id="section-IRIs">
<h3>IRIs</h3>
<p>An <dfn data-lt="iri"><abbr title="Internationalized Resource Identifier">IRI</abbr></dfn>
(Internationalized Resource Identifier) within an RDF graph
is a <a>string</a> that conforms to the syntax
defined in RFC 3987 [[!RFC3987]].</p>
<p>An IRI in the RDF abstract syntax
MUST be <a data-cite="RFC3986#section-5">resolved</a> per [[RFC3986]] and
MUST NOT be a <a data-cite="RFC3986#section-4.2">relative reference</a>.
An IRI MAY contain a <a data-cite="RFC3986#section-3.5">fragment identifier</a>.
An IRI SHOULD follow rules defined by the <a data-cite="RFC3986#section-3.1">IRI scheme</a>.
</p>
<p><dfn>IRI equality</dfn>:
Two IRIs are the same if and only if they consist of the same sequence of
<a data-cite="I18N-GLOSSARY#dfn-code-point" class="lint-ignore">Unicode code points</a>,
as in Simple String Comparison in
<a data-cite="rfc3987#section-5.3.1">section 5.3.1</a> of [[!RFC3987]].
(This is done in the abstract syntax, so the IRIs are resolved
IRIs with no escaping or encoding.)
Further normalization MUST NOT be performed before this comparison. </p>
<p class="note">For convenience, a complete [[ABNF]] grammar
from [[RFC3987]] is provided in <a href="#iri-abnf" class="sectionRef"></a>.</p>
<div class="note" id="note-iris">
<p><strong>URIs and IRIs:</strong>
IRIs are a generalization of
<dfn data-lt="URI" data-lt-noDefault><abbr title="Uniform Resource Identifier">URI</abbr>s</dfn>
[[RFC3986]] that permits a wider range of Unicode characters [[UNICODE]].
Every URI and URL is an IRI, but not every IRI is an URI.
In RDF, IRIs are used as <dfn data-lt="iri reference">IRI references</dfn>, as defined in [[RFC3987]]
<a data-cite="RFC3987#section-1.3">section 1.3</a>.
An IRI reference is common usage of an Internationalized Resource Identifier.
An IRI reference refers to either a resolved <a>IRI</a> or <a>relative IRI reference</a>,
as described by the <em>IRI-reference</em> production in <a href="#iri-abnf" class="sectionRef"></a>.
The abstract syntax uses only fully resolved <a>IRIs</a>.
When IRIs are used in operations that are only
defined for URIs, they must first be converted according to
the mapping defined in
<a data-cite="rfc3987#section-3.1">section 3.1</a>
of [[!RFC3987]]. A notable example is retrieval over the HTTP
protocol. The mapping involves UTF-8 encoding of non-ASCII
characters, %-encoding of octets not allowed in URIs, and
Punycode-encoding of domain names.</p>
<p><strong>URLs:</strong>
The [[[URL]]] is largely compatible with [[RFC3987]] IRIs,
but is based on a processing model important for implementation
within web browsers and are not described using an [[ABNF]] grammar.
</p>
<p><strong>Relative IRI references:</strong>
Some <a>concrete RDF syntaxes</a> permit
<span id="dfn-relative-iris"><!-- obsolete term--></span>
<dfn id="dfn-relative-iri" data-lt="relative iri reference">relative IRI references</dfn> as a convenient shorthand
that allows authoring of documents independently from their final
publishing location.
Relative IRI references must be
<a data-cite="rfc3986#section-5.2">resolved against</a> a
<dfn class="export">base IRI</dfn>.
Therefore, the RDF graph serialized in such syntaxes is well-defined only
if a <a data-cite="rfc3986#section-5.1">base IRI
can be established</a> [[RFC3986]].</p>
<p><strong>URI Schemes:</strong>
Implementations are encouraged to enforce the scheme-specific rules of
the common schemes, such as the
<a data-cite="RFC7230#section-2.7.1">scheme rules for HTTP/HTTPS</a> and the
<a data-cite="did-core#identifier">DID syntax</a>.
Implementations ignore URI scheme rules
for schemes they do not recognize.
</p>
<p><strong>IRI normalization:</strong>
Interoperability problems can be avoided by minting
only IRIs that are normalized according to
<a data-cite="rfc3987#section-5">Section 5</a>
of [[!RFC3987]].
</p>
<ul>
<li>Use lowercase characters in scheme names.</li>
<li>Only use percent-encoding of characters where required by the IRI syntax.</li>
<li>Omit the HTTP or HTTPS default port; <code>http://example/</code>
is preferred over <code>http://example:80/</code>.</li>
<li>Use uppercase hexadecimal letters within percent-encoding
triplets;"<code>%3F</code>" is preferred over
"<code>%3f</code>".</li>
<li>An empty path in an HTTP IRI
<code>http://example/</code> is preferred over
having no path <code>http://example</code>.</li>
<li>Normalize IRIs to remove "<code>/./</code>" and "<code>/../</code>" in the path
component of an IRI.</li>
<li>Use lowercase characters in domain names.
Note that, while ASCII characters in domain names are case-insensitive,
non-ASCII characters in domain names are case-sensitive [[RFC5890]].
Domains are generally only registered with lowercase letters [[RFC5892]].</li>
<li>Avoid using the
<a href="https://datatracker.ietf.org/doc/html/rfc5890#section-2.3.2.1">A-label</a>
(ASCII, punycode-encoded name]) for Internationalized Domain Names [[RFC5890]] in IRIs.</li>
<li>Use IRIs in Unicode [=Normalization Form C=] [[I18N-Glossary]].</li>
</ul>
</div>
</section>
<section id="section-Graph-Literal">
<h3>Literals</h3>
<p>Literals are used for values such as strings, numbers, and dates.</p>
<p>A <dfn data-local-lt="RDF literal">literal</dfn> in an <a>RDF graph</a> consists of
two, three, or four elements, as follow:</p>
<ol>
<li>a <dfn>lexical form</dfn> consisting of a sequence of
<a data-cite="I18N-GLOSSARY#dfn-code-point" class="lint-ignore">Unicode code points</a> [[!UNICODE]]
which are <a data-cite="I18N-GLOSSARY#dfn-scalar-value">Unicode scalar values</a>,
and therefore do not contain
<a data-cite="I18N-GLOSSARY#dfn-surrogate" class="lint-ignore">Unicode surrogate code points</a></li>
<li>a <dfn>datatype IRI</dfn>, being an <a>IRI</a>
identifying a datatype that determines how the lexical form maps
to a <a>literal value</a></li>
<li>if and only if the <a>datatype IRI</a> is
<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</code>, a
non-empty <dfn>language tag</dfn> as defined by [[!BCP47]]. The
language tag MUST be well-formed according to
<a data-cite="bcp47#section-2.2.9">section 2.2.9</a>
of [[!BCP47]],
and MUST be treated consistently, that is, in a case insensitive manner.
Two language tags are the same if they only differ by case.</li>
<li>if and only if the <a>datatype IRI</a> is
<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code>,
a non-empty <a>language tag</a>
that MUST be well-formed according to <a data-cite="bcp47#section-2.2.9">section 2.2.9</a>
of [[!BCP47]],
and MUST be treated consistently, that is, in a case insensitive manner,
and a <dfn>base direction</dfn> that MUST be either `ltr` or `rtl`.</li>
</ol>
<p>A literal is a <dfn>language-tagged string</dfn> if the third element
is present and the fourth element is not present.
Lexical representations of language tags MAY be case normalized,
(for example, by canonicalizing as defined by
<a data-cite="bcp47#section-4.5">BCP 47 section 4.5</a>).
</p>
<p>A literal is a <dfn id="dfn-dir-lang-string">directional language-tagged string</dfn>
if both the third element and fourth elements are present.
The third element, the language tag, is treated identically as in a <a>language-tagged string</a>,
and the fourth element, <a>base direction</a>, MUST be either `ltr` or `rtl`, which MUST be in lower case.</p>
<p>The meanings of the <a>base direction</a> values are:</p>
<ul>
<li>`ltr`: indicates that the initial text direction is set to left-to-right.</li>
<li>`rtl`: indicates that the initial text direction is set to right-to-left.</li>
</ul>
<p>Please note that concrete syntaxes MAY support
<dfn data-lt="simple literal" class="export">simple literals</dfn> consisting of only a
<a>lexical form</a> without any <a>datatype IRI</a>, <a>language tag</a>, or <a>base direction</a>.
Simple literals are syntactic sugar for abstract syntax
<a>literals</a>
with the <a>datatype IRI</a>
<code>http://www.w3.org/2001/XMLSchema#string</code>
(which is commonly abbreviated as <code>xsd:string</code>).
Similarly, most concrete syntaxes represent
<a>language-tagged strings</a> and <a>directional language-tagged strings</a> without
the <a>datatype IRI</a> because it always equals either
<code>http://www.w3.org/1999/02/22-rdf-syntax-ns#langString</code> (<code>rdf:langString</code>)
or <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#dirLangString</code> (<code>rdf:dirLangString</code>), respectively.</p>
<p>The <dfn>literal value</dfn> associated with a <a>literal</a> is:</p>
<ul>
<li>If the literal is a <a>language-tagged string</a>,
then the literal value is a pair consisting of its <a>lexical form</a>
and its <a>language tag</a>, in that order.</li>
<li>If the literal is a <a>directional language-tagged string</a>, then the literal value is
a tuple of its <a>lexical form</a>, its <a>language tag</a>, and its <a>base direction</a>,
likewise in that order.</li>
<li>If the literal's <a>datatype</a> is handled by an RDF implmentation,
<ul>
<li>if the literal's <a>lexical form</a> is in the <a>lexical space</a>
of the <a>datatype</a>, then the literal value is the result of applying
the <a>lexical-to-value mapping</a> of the datatype to the
<a>lexical form</a>.</li>
<li>otherwise, the literal is <dfn data-lt-no-plural>ill-typed</dfn> and no literal value can be
associated with the literal. Such a case produces a semantic
inconsistency but is not <em>syntactically</em> ill-formed.
Implementations SHOULD accept [=ill-typed=] literals and produce RDF
graphs from them. Implementations MAY produce warnings when
encountering [=ill-typed=] literals.</li>
</ul>
</li>
<li>If the literal's <a>datatype IRI</a> is <em>not</em>
handled by an RDF implementation, then the literal value is
not defined by this specification. Implementations SHOULD accept
literals with unknown datatype IRIs and produce RDF graphs from them.
</li>
</ul>
<p><dfn data-local-lt="term-equal">Literal term equality</dfn>:
Two literals are term-equal (the same <a>RDF literal</a>)
if and only if:</p>
<ul>
<li>the two <a>lexical forms</a> compare equal</li>
<li>the two <a>datatype IRIs</a> compare equal</li>
<li>the two <a>language tags</a> (if any) compare equal</li>
<li>the two <a>base directions</a> (if any) compare equal</li>
</ul>
<p>Comparison is performed using
<a data-cite="I18N-GLOSSARY#dfn-case-sensitive">case sensitive matching</a>
(see description of string comparison in
<a href="#rdf-strings" class="sectionRef"></a>)
except for language tags, where the comparison is performed using
<a data-cite="I18N-GLOSSARY#dfn-case-sensitive">ASCII case-insensitive matching</a>.
Thus, two literals can have the same value
without being the same <a>RDF term</a>.
For example:</p>
<pre>
"1"^^xs:integer
"01"^^xs:integer
</pre>
<p>denote the same <a data-lt="literal value">value</a>, but are not the
same literal <a>RDF terms</a> and are not
<a>term-equal</a> because their
<a>lexical forms</a> differ.</p>
</section>
<section id="section-blank-nodes">
<h3>Blank Nodes</h3>
<p><dfn data-lt="blank node">Blank nodes</dfn> are disjoint from
<a>IRIs</a> and <a>literals</a>. Otherwise,
the set of possible blank nodes is arbitrary. RDF makes no reference to
any internal structure of blank nodes.</p>
<div class="note" id="note-bnode-id">
<p><span id="dfn-blank-node-identifiers"><!-- obsolete term--></span><dfn data-lt="blank node identifier">Blank node identifiers</dfn>
are local identifiers that are used in some
<a>concrete RDF syntaxes</a>
or RDF store implementations.
They are always locally scoped to the file or RDF store,
and are <em>not</em> persistent or portable identifiers
for blank nodes. Blank node identifiers are <em>not</em>
part of the RDF abstract syntax, but are entirely dependent
on the concrete syntax or implementation. The syntactic restrictions
on blank node identifiers, if any, therefore also depend on
the concrete RDF syntax or implementation. Implementations that handle blank node
identifiers in concrete syntaxes need to be careful not to create the
same blank node from multiple occurrences of the same blank node identifier
except in situations where this is supported by the syntax.</p>
<p>The term "blank node label" is sometimes used informally
as an alternative to the term <a>blank node identifier</a>.
This alternative was also used in earlier versions of
some RDF-related specifications such as [[SPARQL11-QUERY]].
In the interest of consistency, the use of this alternative
term is discouraged now.</p>
</div>
</section>
<section id="section-triple-terms">
<h3>Triple Terms</h3>
<p>An [=RDF triple=] used as the [=object=] of another [=triple=] is called a <dfn class=export >triple term</dfn>.
In a given [=RDF graph=], a [=triple=] can appear as a [=triple term=], an [=asserted triple=], or both.
</p>
<p>The set of [=RDF terms=] <dfn class=export data-lt="appear">appearing</dfn> in an [=RDF triple=] |t| is defined inductively as follows:</p>
<ul>
<li>The [=subject=], [=predicate=] and [=object=] of |t| [=appear=] in |t|.
<li>If a [=triple term=] |t2| appears in |t|, then any [=RDF term=] [=appearing=] in |t2| also [=appears=] in |t|.
</ul>
<p>By extension, an [=RDF term=] is said to [=appear=] in an [=RDF graph=] if it appears in an [=asserted triple=] of that graph. An [=RDF triple=] is said to [=appear=] in an [=RDF graph=] if it is either an [=asserted triple=] of that graph or a [=triple term=] [=appearing=] in that graph.</p>
<p class="note">Every <a>triple</a> with a <a>triple term</a> as its [=object=] SHOULD
use <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#reifies</code> (<code>rdf:reifies</code>)
as its <a>predicate</a>.
Every <a>triple</a> whose <a>object</a> is not a <a>triple term</a> SHOULD NOT
use <code>http://www.w3.org/1999/02/22-rdf-syntax-ns#reifies</code> (<code>rdf:reifies</code>)
as its <a>predicate</a>.</p>
</section>
<section id="graph-isomorphism">
<h3>Graph Comparison</h3>
<p id="section-graph-equality">Two
<a>RDF graphs</a> <var>G</var> and <var>G'</var> are
<dfn data-lt="graph isomorphism|isomorphic" data-lt-noDefault class="export">isomorphic</dfn>
(that is, they have an identical form)
if there is a bijection <var>M</var>
from the set of all <a>RDF terms</a> into that same set,
such that all of the following properties hold:</p>
<ul>
<li><var>M</var> maps blank nodes to blank nodes.</li>
<li><var>M</var>(<var>lit</var>)=<var>lit</var> for every <a>RDF literal</a> <var>lit</var>.</li>
<li><var>M</var>(<var>iri</var>)=<var>iri</var> for every <a>IRI</a> <var>iri</var>.</li>
<li><var>M</var>(<var>tt</var>) is the triple term ( <var>M</var>(<var>s</var>), <var>M</var>(<var>p</var>), <var>M</var>(<var>o</var>) ) if <var>tt</var> is a triple term of the form ( <var>s</var>, <var>p</var>, <var>o</var> ).</li>
<li>The triple ( <var>s</var>, <var>p</var>, <var>o</var> ) is in <var>G</var> if and
only if the triple ( <var>M</var>(<var>s</var>), <var>M</var>(<var>p</var>), <var>M</var>(<var>o</var>) ) is in
<var>G'</var>.</li>
</ul>
<p>See also: <a>IRI equality</a>, <a>literal term equality</a>.</p>
<p>With this definition, <var>M</var> shows how each blank node
in <var>G</var> can be replaced with
a new blank node to give <var>G'</var>. Graph isomorphism
is needed to support the RDF Test Cases [[RDF11-TESTCASES]] specification.</p>
</section>
<section id="section-text-direction" class="informative">
<h3>Initial Text Direction</h3>
<p>The <a>base direction</a> of a <a>directional language-tagged string</a>
provides a means of establishing the initial direction of text,
including text which is a mixture of right-to-left and left-to-right scripts.
The [=Unicode Bidirectional Algorithm=] [[?I18N-Glossary]] provides support for automatically rendering
a sequence of characters in logical order,
so that they are visually ordered as expected,
but this is not sufficient to correctly render bidirectional text.</p>
<p>For example, some text with a language tag "`he`" might be displayed
in a left-to-right context (such as an English web page) as
<bdi dir="ltr">פעילות הבינאום, W3C</bdi>, which is incorrect. When provided
to a user agent including base direction information (such as using
HTML's `dir` attribute) it can then be correctly presented as:
</p>
<div lang="he" dir="rtl">פעילות הבינאום, W3C</div>
<p>In the absence of an explicit initial text direction,
the [=Unicode Bidirectional Algorithm=] detects the text direction from the content.
This depends on the first strongly directional character in the text
or on the context.
To avoid [=spillover effects=], users need to employ [=bidi isolation=]
whenever text is inserted into a larger document.
For example,
"<code><bdi lang="he">ספרים בינלאומיים!</bdi></code>"
displays the Hebrew characters in a right-to-left fashion
— i.e., as "<bdi lang="he">ספרים בינלאומיים!</bdi>"
— while they would be stored in memory as
"<bdo dir="ltr" lang="he">ספרים בינלאומיים!</bdo>"</p>
</section>
<section id="section-skolemization">
<h3>Replacing Blank Nodes with IRIs</h3>
<p>Blank nodes do not have identifiers in the RDF abstract syntax. The
<a>blank node identifiers</a> introduced
by some concrete syntaxes have only
local scope and are purely an artifact of the serialization.</p>
<p>In situations where stronger identification is needed, systems MAY
systematically replace some or all of the blank nodes in an RDF graph
with <a>IRIs</a>. Systems wishing to do this SHOULD
mint a new, globally
unique IRI (a <dfn class="export">Skolem IRI</dfn>) for each blank node so replaced.</p>
<p>This transformation does not appreciably change the meaning of an
RDF graph, provided that the Skolem IRIs do not occur anywhere else.
It does however permit the possibility of other graphs
subsequently using the Skolem IRIs, which is not possible
for blank nodes.</p>
<p>Systems may wish to mint Skolem IRIs in such a way that they can