example34d.tex

\documentclass[oneside]{book}
\usepackage{kantlipsum}
\usepackage{boxedminipage}

\usepackage[twoside]{fancyhdr}
\pagestyle{fancy}
\fancyhf{}
\setlength{\headheight}{35pt}
\fancyhead[LE,RO]{\nouppercase{\rightmark}} % Section title
\fancyhead[LO,RE]{\nouppercase{\leftmark}}  % Chapter title
\fancyfoot[LE,RO]{\thepage}
\fancyheadwidth[L,R]{0.48\headwidth}

\begin{document}

\thispagestyle{plain}
\tableofcontents

\bigskip

\noindent
\begin{boxedminipage}{\textwidth}
This is example 34d in the \texttt{fancyhdr} documentation.

This document has some very long chapter and section titles. We use the \verb|\fancyheadwidth| command to fit the titles each in their own half of the header (actually slightly less than half). This command is available in \texttt{fancyhdr} version 5.0 and later.

\begin{verbatim}
\setlength{\headheight}{35pt}
\fancyhead[LE,RO]{\nouppercase{\rightmark}} % Section title
\fancyhead[LO,RE]{\nouppercase{\leftmark}}  % Chapter title
\fancyfoot[LE,RO]{\thepage}
\fancyheadwidth[L,R]{0.48\headwidth}
\end{verbatim}
\end{boxedminipage}

\newpage
\pagestyle{fancy}
\chapter{This is a very long chapter title to see if we can
          let fancyhdr fit it in the header}
\thispagestyle{fancy}

\kant[2-4]

\section{Normal section title}

\kant[5-6]

\section{This is a very long section title that will not fit
         in the header}

\kant[7]

\end{document}