In music, flat means lower in pitch. The flat symbol, , indicates that the note to which it is applied is played one chromatic semitone lower. The opposite of flat is sharp, indicating a raising of pitch.
The flat symbol () appears in key signatures to indicate which notes are flat throughout a section of music, and also in front of individual notes as an accidental, indicating that the note is flat until the next bar line.
Pitch change
The symbol is a stylised lowercase b, derived from Italian be molle for "soft B" and German blatt for "planar, dull". It indicates that the note to which it is applied is played one chromatic semitone lower. In the standard modern tuning system, 12-tone equal temperament, this corresponds to 100 cents.
The difference in pitch indicated by a sharp or flat varies across different tuning systems. In tuning systems from the 16th and 17th century, and in modern microtonal tunings, the difference is normally smaller than a standard semitone. The change in pitch in the old quarter-comma meantone system would be 76.05 cents, in just intonation it would be 70.57 cents, and in Pythagorean tuning 113.7 cents. In well temperaments, the change in pitch indicated by a sharp or flat can vary.
Intricate systems of microtuning may replace the standard flat or sharp with different symbols for raising and lowering pitch. In 53 equal temperament tuning sharps and flats have two or three different sub-levels, and notation for flattening notes varies, but usually involves several different symbols; one of the sets of flat symbols is (67.9 cents), (45.3 cents), and (22.6 cents), used both separately and in combinations.
Related symbols
A double flat () lowers a note by two chromatic semitones (a whole step in 12-tone equal temperament).
:<score>{ \omit Score.TimeSignature \relative c { beses1 } }</score>
A quarter-tone flat, half flat or demiflat indicates the use of quarter tones; it may be marked with various symbols including a flat with a slash (), a flat with a 4 (), or a (). A three-quarter-tone flat, flat and a half or sesquiflat is represented by a demiflat and a whole flat (). The symbols , , , among others, represent comma flat or eighth-tone flat.
:<score>{ \omit Score.TimeSignature \relative c { ceh1 deseh } }</score>
A triple flat () is very rare. As expected, it lowers a note by three chromatic semitones (a whole tone and semitone in 12-tone equal temperament). (For example, B is enharmonic with A.)
:<score>{
\omit Score.TimeSignature \relative c {
\tweak Accidental.stencil #ly:text-interface::print \tweak Accidental.text \markup { \concat { \flat \doubleflat beses1
} }</score>
Unicode
The Unicode character ♭ (U+266D) can be found in the block Miscellaneous Symbols; its HTML entity is Other assigned flat signs can be found in the Musical Symbols block and are as follows: