# Changing Chapter Number to have Two Numbers

I am working in the book class, and currently I have

\setcounter{chapter}{10}

\renewcommand{\chaptername}{Lecture}


Which generates

Lecture 11

How do I make it say,

Lecture 11-12

or

Lecture 11,12

• Welcome to TeX.sx! On what basis the double number should be used? – egreg Mar 10 '13 at 0:08
• do you always want n-(n+1) and the chapter number to increment by 2 so next would be 13-14 or just this as a special case combined chapter? – David Carlisle Mar 10 '13 at 0:09
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A simplistic way, if you are going to manually set the counter is this:

\documentclass{book}
\setcounter{chapter}{10}
\renewcommand{\chaptername}{Lecture}
\makeatletter
\def\thechapter{\@arabic\c@chapter--\the\numexpr\c@chapter+1\relax}
\makeatother
\begin{document}
\chapter{Intro to Maths}
\end{document}


If you are going to turn the lecture notes into something more complicated, you may have to hack the \@makechapterhead and \@makeschapterhead macros, also you will need to re-write code for the toc, figure and table captions and headings etc. I would rather stick to the standard way of one lecture one chapter and rather have a front page (hack the Part to do the combination).

• \c@chapter is a counter so the \@arabic is not necessary. If anything, then \def\thechapter{\@arabic\c@chapter--\@arabic\numexpr\c@chapter+1}. – Qrrbrbirlbel Mar 10 '13 at 2:54
• @Qrrbrbirlbel Thanks you right. I would rather then just leave the \the. Probably also better to inc the counter as well at this point. I wasn't sure what the OP was trying to do. – Yiannis Lazarides Mar 10 '13 at 3:09
• better to use an en-dash between numbers. – barbara beeton Mar 10 '13 at 12:46
• Why not simply \renewcommand{\thechapter}{11--12}? – egreg Mar 10 '13 at 15:55