= TeX control sequence
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A \b[TeX control sequence] is a named command represented by a command token. With ordinary category conventions, a <TeX control word> begins with the escape character and continues through letters; a <TeX control symbol> uses a single nonletter after the escape.<TeX by Topic by Victor Eijkhout>[§2.7.1: control-word and control-symbol scanning]{ref}
Thus `\demo` is normally a control word and `\%` a control symbol. The name does not require a particular implementation: its current meaning may be a <TeX primitive> or a <TeX macro>. For example, `\def\demo{A}\let\saved=\demo\def\demo{B}` leaves `\demo` expanding to `B` and `\saved` expanding to the saved definition `A`.
Spaces also illustrate why the tokenization rules matter. Under the usual conventions, the space after a control word delimits its name and is ignored by input processing. Printing a space may therefore require an explicit command or another appropriate input construction. The <TeX category code> assignments determine which characters count as letters during this scan.
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