In music it’s a flat, not a lowercase ‘B’, although I keep writing it as a ‘b’ in my posts (don’t blame me, it’s just faster)… Symbol: ♭. It’s a note (any note) minus a semitone, ie. it’s the opposite of a sharp. For instance, A flat is note A, minus a semitone. On a piano keyboard, it’s the black key located to the A’s left. On a guitar’s fretboard, it’s a fret further away from the guitar’s body. Also known as: bemolle (deriving from Italian).