= TeX parameter token
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A \b[TeX parameter token] marks an argument position in a <TeX macro> definition. With ordinary category assignments, `#1` through `#9` name the available positions. During <TeX expansion>, the supplied argument tokens replace the corresponding placeholders.<TeX by Topic by Victor Eijkhout>[§§2.4 and 11.5: parameter tokens and argument substitution]{ref}
For an original example, `\def\twice#1{#1:#1}` followed by `\twice{ab}` expands to `ab:ab`. The braces delimit one argument containing two character tokens; they are not copied into the replacement at each placeholder. Calling `\twice7` instead gives `7:7`. Thus `#1` identifies an argument slot rather than instructing <TeX> to print the digit 1.
The definition's replacement list can be inspected before execution. Its placeholders remain distinct from ordinary digit tokens, even when an actual argument happens to be a digit. This separation makes token substitution useful for reusable notation.
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