flat
An accidental that lowers a note by one semitone. Notated as a lowercase b.
In Depth
A flat lowers a note by one semitone — the smallest standard interval in Western music. The flat symbol (♭) is placed before a note on the staff, and once written, it applies to all subsequent occurrences of that note within the same measure unless cancelled by a natural sign.
Flats also appear in key signatures, where they indicate the prevailing key of a piece. Keys with flats include F major (one flat), B♭ major (two flats), and so on up to seven flats. Double flats (𝄫) lower a note by two semitones and appear occasionally in chromatic passages and enharmonic modulations.
The flat symbol (♭) evolved from the letter b — in medieval notation, b molle (soft b) meant B-flat, while b durum (hard b) meant B-natural, which eventually became the natural sign.