Hentaigana
Hentaigana (変体仮名) is the term for alternative kana letterforms equivalent to standard kana characters. This is the legacy of manyogana, where multiple kanji could be used to represent one syllable; as these became simplified into cursive forms, hentaigana were the result. Hentaigana were used more or less interchangeably with their equivalent hiragana on an ad hoc, individual basis until 1900, when the hiragana syllabary was standardized to one character per mora.
The hiragana "syllabic n" (ん) originally derives from a hentaigana for /mu/. The spelling reform of 1900 separated the two uses, declaring that む could only be used for /mu/ and ん could only be used for syllable-final /n/. Previously, in the absence of a character for the syllable-final /n/, the sound was spelled (but not pronounced) identically to /mu/, and readers had to rely on context to determine what was intended.
Hentaigana are considered obsolete in modern writing, but a few uses still remain. For example, many soba shops use hentaigana to spell kisoba on their signs.