# Is it \catcode or \catcode`?

I've noticed that `\catcode` is poorly documented in the TeXbook. The primitive is defined as `\catcode` but every time it is used, it is used as `\catcode`` ... the backquote is never discussed. ``` itself is not active (right?) so I don't understand its function. Is it required for `\catcode` to work properly?

The backquote notation is discussed in the TeXbook, page 44.

`\catcode` takes as its first argument the character code of the character you want to change the catcode of. For example,

``````\catcode65=\active
``````

will make ‘A’ an active character.

Instead of inserting the number ‘65’ directly, TeX allows the character code of a character to be inserted with the backtick notation you're referring to:

``````\catcode`A=\active
``````

You need to escape the character if it happens to have a special catcode, though: ``\%`, for instance.

This backtick notation can be used wherever TeX expects a number. E.g.,

``````\number`A
``````

will typeset ‘65’.

• Beat me to it by moments, Will. Just to add, this is described in the TeXbook, page 44. Sep 21, 2010 at 1:59
• Feel free to fix up any of my clunky statements. My explaining-brain feels like mush today. Sep 21, 2010 at 2:23
• I guess it should read `\catcode'A=\active` in your example (of course with a backtick; don't know how to put a backtick into the middle of a code sample.) Sep 21, 2010 at 7:45
• Oh yes, that was quoted incorrectly. Thanks for spotting the error. Sep 21, 2010 at 11:30
• Thanks for that informative explanation. To recap: in situations where TeX is looking for a number, it will interpret ` as "the character code of the next character". You have to admit, sometimes it's hard to navigate the TeXbook! Sep 21, 2010 at 21:08