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Anyone who has ever browsed through either the plain format or the LaTeX format will have noticed that a lot of control sequences contain an 'at' sign: @. These are control sequences that are meant to be inaccessible to the ordinary user.

Near the beginning of the format files the instruction

\catcode‘@=11

occurs, making the at sign into a letter, meaning that it can be used in control sequences. Somewhere near the end of the format definition the at sign is made 'other' again:

\catcode‘@=12

Now why is it that users cannot call a control sequence with an at sign directly, although they can call macros that contain lots of those 'at-definitions'? The reason is that the control sequences containing an @ are internalized by TeX at definition time, after which they are a token, not a string of characters. Macro expansion then just inserts such tokens, and at that time the category codes of the constituent characters do not matter any more.

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