TeX/catcode

< TeX 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~{...}