 TeX input processor

The TeX input processor converts input characters into TeX tokens using their current TeX category codes. Its whitespace handling is described by TeX input states: new line N, skipping spaces S and middle of line M.^[ref]

Under ordinary conventions, the invented input A B produces an A character token, one space token and a B character token. After A, state M allows the first space through and switches to S; the next two spaces disappear. Reading B returns to M. A space-only line begins in N and its end produces \par, whereas the end of an ordinary text line in M supplies a space.^[ref]

This explains why source indentation and repeated spaces need not survive as visible word spacing. The complete scanner also handles comments, TeX control sequences and character-code conversions; the N/S/M diagram describes the whitespace part of the process.