Space length between the chapter number and the chapter title

Question

How can I learn the horizontal distance between the chapter number and the chapter title both in ToC and the body text where I use \documentclass[12pt,reqno]{book}. Which keys hold these values?

Answer 1

book (like report) sets the ToC entry for a chapter using \l@chapter. This is what it looks like:

\newcommand*\l@chapter[2]{%
  \ifnum \c@tocdepth >\m@ne
    \addpenalty{-\@highpenalty}%
    \vskip 1.0em \@plus\p@
    \setlength\@tempdima{1.5em}%
    \begingroup
      \parindent \z@ \rightskip \@pnumwidth
      \parfillskip -\@pnumwidth
      \leavevmode \bfseries
      \advance\leftskip\@tempdima
      \hskip -\leftskip
      #1\nobreak\hfil \nobreak\hb@xt@\@pnumwidth{\hss #2}\par
      \penalty\@highpenalty
    \endgroup
  \fi}

The specific thing to note here is the value of \@tempdima, set to 1.5em. The first argument of \l@chapter (or #1) uses \numberline, which is defined in the kernel latex.ltx as:

\def\numberline#1{\hb@xt@\@tempdima{#1\hfil}}

creating a box of width \@tempdima and left-aligning its contents. Note that this 1.5em is based on the prevailing font at the time of setting (\normalsize\bfseries by default).

---

The (numbered) chapter head in book (and report) is set via the macro \@makechapterhead. Here's a take on that:

\def\@makechapterhead#1{%
  \vspace*{50\p@}%
  {\parindent \z@ \raggedright \normalfont
    \ifnum \c@secnumdepth >\m@ne
      \if@mainmatter
        \huge\bfseries \@chapapp\space \thechapter
        \par\nobreak
        \vskip 20\p@
      \fi
    \fi
    \interlinepenalty\@M
    \Huge \bfseries #1\par\nobreak
    \vskip 40\p@
  }}

The space between the word Chapter (or \@chapapp in the definition above) and the number (\thechapter in the definition above) is a regular \space or \.

---

In both instances, if you want to change these defaults, you have at least 3 options:

1. Use a package that allows you to modify this content (like titlesec or tocloft or the like);

2. Copy the entire definition and change it to your liking (making \newcommand -> \renewcommand);

3. Use etoolbox to patch the command. For example,

   \usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
   \makeatletter
   \patchcmd{\l@chapter}% <cmd>
     {1.5em}% <search>
     {5.5em}% <replace>
     {}{}% <success><failure>
   \makeatother
   

   would use a 5.5em wide box for setting the chapter number in the ToC, increasing the gap between the number and title. Also

   \usepackage{etoolbox}% http://ctan.org/pkg/etoolbox
   \makeatletter
   \patchcmd{\@makechapterhead}% <cmd>
     {\space}% <search>
     {\hspace*{100pt}\ignorespaces}% <replace>
     {}{}% <success><failure>
   \makeatother
   

   would insert a 100pt gap between the word Chapter (or whatever \@chapapp is) and the number. \ignorespaces would just remove any trailing spaces left inside the original definition of \@makechapterhead.