Originally a lowercaseb written with a round loop (the so called "round B" or "B rotundum") to distinguish from a lowercaseb written with a square loop ("square B" or "B quadratum"), when the only notes used in (Western) music were what are now considered C, D, E, F, G, A, B-flat and B. The round B was used for the former and the square B for the latter. The round B was then later generalized as the symbol for flat notes.
♭
- (music) Aflat; abemol (a note onesemitone lower).
- In general typography, the symbol is usually written as lowercaseb.
Flat (music) on Wikipedia.Wikipedia