I must define number be for subsubsection form :

\chapter : 1

\section : 1.1

\subsection : 1.1.1

\subsubsection : 1.1.1.a

Can you help me define form.


The numbering of subsections in the book document class should already be in the format you require. To modify \thesubsubsection to show the final "number" in alphabetical rather than arabic-numeral format, you could issue the commands


in the preamble of your document.

As you can probably tell from this definition, \c@subsubsection is a counter variable, \@alph instructs TeX to display the value of the counter in lowercase-alphabetical format (a, b, c, etc), and \thesubsection. instructs TeX to prepend the (already-defined) representation of the subsection counter, followed by a dot (.), to \@alph\c@subsubsection.

As @ThorstenDonig has pointed out in a comment, the preceding commands -- which are fairly low-level -- can be replaced with the following, higher-level command (which is especially nice at it obviates the need to use \makeatletter and \makeatother):

  • 2
    Why so complicated? \renewcommand{\thesubsubsection}{\thesubsection.\alph{subsubsection}} doesn't need \makeatletter and \makeatother. – Thorsten Donig Nov 22 '13 at 14:38
  • I concur with Thorsten: the user level command \alph is there just for this purpose. – egreg Nov 22 '13 at 15:40
  • @ThorstenDonig - Because \alph{<counter>} is defined in terms of \@alph and \c@<counter> and because I like using low-level commands... – Mico Nov 22 '13 at 16:13
  • Low level commands are for the advanced user. For beginners they are only confusing. And I don't think that the "normal" user is interested in low level commands. – Thorsten Donig Nov 22 '13 at 17:31
  • @ThorstenDonig - Feel free to downvote my answer. – Mico Nov 22 '13 at 18:09

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