When a flat (♭) is applied to a note in music, it lowers the pitch of that note by one half-step (or semitone). This means that the note will sound one half-step lower than the natural note (the note without any accidentals). For example, if you flatten a B natural by adding a flat symbol next to it (B♭), it will sound a half-step lower, at the pitch A♯ (which is enharmonically equivalent to B♭ on most modern instruments tuned in equal temperament). Flats are one of several types of accidentals used in musical notation to alter the pitches of notes.
