allograph

1. A variant shape of a letter.
2. A letter or combination of letters that can represent one phoneme, as f and gh can represent the phoneme /f/.
3. Writing, especially a signature, made by one person for another.

A writing or signature made by one person for another.

When a principal gives his or her agent the power to pay creditors, the checks written by the agent are allographs for the principal.

An autograph is the opposite of an allograph.

The noun has 2 meanings:

Meaning #1: a variant form of a grapheme as `m' or `M'

Meaning #2: a signature made by one person for another