TeX control symbol

A TeX control symbol is a TeX control sequence formed by an escape followed by one character whose TeX category code is not letter. Under ordinary conventions, \% is one such command.^[ref]

For an invented example, define \def\!{Q} and then read \! X. The resulting replacement starts with Q, one ordinary space token and X. The nonletter ends the command name immediately; the first following space is processed in state M and survives. Compare \def\word{Q} followed by \word X, which produces QX.

Control space, written as an escape followed by a space, has special handling: it enters state S. Thus a rule that discards every space after every command would predict the wrong result for the \! example.^[ref]