Flat (music)

In music, flat, or (Italian: "soft B") means "lower in pitch". In music notation, the flat symbol, ♭, derived from a stylised lowercase "b", lowers a note by a half step (semitone).Intonation or tuning is said to be flat when it is below the true pitch.

Flat accidentals are used in the key signatures of F major/D minor, B♭ major/G minor, E♭ major/C minor, A♭ major/F minor, D♭ major/B♭ minor, and the less frequently used keys of G♭ major/E♭ minor, C♭ major/A♭ minor. The order of flats in the key signatures of music notation, following the circle of fifths, is B♭, E♭, A♭, D♭, G♭, C♭, and F♭. A mnemonic for this is: Before Eating A Doughnut Get Coffee First.

...
Wikipedia