TeX/catcode

Synopsis

\catcode`\[char]=[num]

Description

\catcode is the command that's used to change the category code of a character. The category code tells TeX what to do when it encounters that character later in the text. A general assignment looks something like

\catcode`\@=11

which defines the @ character to be a letter.

The category codes in TeX are as follows:

0 = Escape character, normally \
1 = Begin grouping, normally {
2 = End grouping, normally }
3 = Math shift, normally $
4 = Alignment tab, normally &
5 = End of line, normally <return>
6 = Parameter, normally #
7 = Superscript, normally ^
8 = Subscript, normally _
9 = Ignored character, normally <null>
10 = Space, normally <space> and <tab>
11 = Letter, normally only contains the letters a,...,z and A,...,Z. These characters can be used in command names
12 = Other, normally everything else not listed in the other categories
13 = Active character, for example ~
14 = Comment character, normally %
15 = Invalid character, normally <delete>

Using category codes, it is possible to completely reconfigure TeX and allows support for international users.

A character of code 13 is called active. In a sense it is treated as if it were a command sequence in of itself, and definitions can be given in the normal fashion. Hence to define ~, Plain TeX has

\def~{...}