Guide to LaTeX (4th Edition) (Tools and Techniques

56 Chapter 3. Document Layout and Organization
n is the current chapter number. At this point in the present book, we are
in Chapter 3, Section 3.3, Subsection 3.3.3.
For each sectioning command there is an internal counter that is
incremented by one every time that command is called, and reset to zero
on every call to the next higher sectioning command. These counters are
not altered by the *-forms, a fact that can lead to difficulties if standard
and *-forms of the commands are mixed such that the *-forms are higher
in the hierarchy than the standard forms. There are no problems, however,
if the *-forms are always lower than the standard forms. The sequence
\section ... \subsection ... \subsubsection* ...
numbers the headings for \section and \subsection while leaving the
headings for \subsubsection without any numbering.
The sectioning command \part is a special case and does not affect
the numbering of the other commands.
The automatic numbering of sections means that the numbers might
not necessarily be known at the time of writing. The author may be writing
them out of their final order, or might later introduce new sections or even
remove some. If he or she wants to refer to a section number in the text,
some mechanism other than typing the number explicitly will be needed.
The L AT E X cross-reference system, described in detail in Section 9.2.1,
accomplishes this task with the two basic commands
\label{name} \ref{name}
the first of which assigns a keyword name to the section number, while the
second may be used as reference in the text for printing that number. The
keyword name may be any combination of letters, numbers, or symbols.
For example, in this book the command \label{sec:xref} has been
typed in at the start of Section 9.2.1, so that this sentence contains the
input text at the start of Section \ref{sec:xref}.
A second referencing command is \pageref for printing the page
number where the corresponding \label is defined.
The referencing commands may be used in many other situations for
labeling items that are numbered automatically, such as figures, tables,
equations.
Every sectioning command is assigned a level number such that \section
! is always level 1, \subsection level 2, . . . \subparagraph level 5. In document
class article, \part is level 0 while in book and report classes, \part is level
−1 and \chapter becomes level 0. Section numbering is carried out down to
the level given by the number secnumdepth. This limit is set to 2 for book
and report, and to 3 for article. This means that for book and report, the
section numbering extends only to the level of \subsection and for article to
\subsubsection.
In order to extend (or reduce) the level of the section numbering, it is necessary
to change the value of secnumdepth. This is done with the command
\setcounter{secnumdepth}{num}