In music, flat (Italian bemolle for "soft B") means "lower in pitch". Flat is the opposite of sharp, which is a raising of pitch. In musical notation, flat means "lower in pitch by one semitone (half step)", notated using the symbol ♭ which is derived from a stylised lowercase 'b'.  For instance, the music below has a key signature with three flats (indicating either E♭ major or C minor) and the note, D♭, has a flat accidental.
Under twelve-tone equal temperament, D♭ for instance is enharmonically equivalent to C♯, and G♭ is equivalent to F♯. In any other tuning system, such enharmonic equivalences in general do not exist. To allow extended just intonation, composer Ben Johnston uses a sharp as an accidental to indicate a note is raised 70.6 cents (ratio 25:24), and a flat to indicate a note is lowered 70.6 cents.
In intonation, flat can also mean "slightly lower in pitch" (by some unspecified amount). If two simultaneous notes are slightly out-of-tune, the lower-pitched one (assuming the higher one is properly pitched) is "flat" with respect to the other. Furthermore, the verb flatten means to lower the pitch of a note, typically by a small musical interval.
Flats are used in the key signatures of
- F major / D minor (B♭)
- B♭ major / G minor (adds E♭)
- E♭ major / C minor (adds A♭)
- A♭ major / F minor (adds D♭)
- D♭ major / B♭ minor (adds G♭)
- G♭ major / E♭ minor (adds C♭)
- C♭ major / A♭ minor (adds F♭)
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♭ (mnemonics for which include Battle Ends And Down Goes Charles' Father and Before Eating A Doughnut Get Coffee First).
Double flats also exist, which look like (similar to two flats, ♭♭) and lower a note by two semitones, or a whole step. The Unicode character 𝄫 (U+1D12B) in the Musical Symbols block represents the double-flat sign.
A quarter-tone flat or half flat, indicating the use of quarter tones, may be marked with various symbols including a flat with a slash ( ) or a reversed flat sign ( ). A three-quarter-tone flat, flat and a half or sesquiflat, is represented by a half flat and a regular flat ( ).
- Benward & Saker (2003). Music in Theory and Practice, Vol. 1, p. 6. McGraw-Hill, Seventh edition. "Flat (♭)—lowers the pitch a half step."
- Flat, Glossary, Naxos Records
- John Fonville. "Ben Johnston's Extended Just Intonation- A Guide for Interpreters", p. 109, Perspectives of New Music, Vol. 29, No. 2 (Summer, 1991), pp. 106–137. "...the 25/24 ratio is the sharp (♯) ratio ... this raises a note approximately 70.6 cents."
- Byrd, Donald (September 2016). "Extremes of Conventional Music Notation". Indiana University Bloomington. Retrieved 4 November 2016.
- Media related to Flats (music) at Wikimedia Commons