In music, flat, or Bemolle, means "lower in pitch" and the flat symbol lowers a note by a half step. Intonation may be flat, sharp, or both, successively or simultaneously. More specifically, in music notation, flat means, "lower in pitch by a semitone (half step)," and has an associated symbol (♭), which is a stylised lowercase "b"  that may be found in key signatures or as an accidental, as may sharps. The Unicode character '♭' (U+266D) is the flat sign. Its HTML entity is ♭.
Under twelve tone equal temperament, C flat for instance is the same as, or enharmonically equivalent to, B natural, and G flat is the same as F sharp. Note that 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), or a flat to indicate a note is lowered 70.6 cents.
Double flats also exist, which look like and lower a note by two semitones, or a whole step. Less often (in for instance microtonal music notation) one will encounter half, or three-quarter, or otherwise altered flats. The Unicode character '𝄫' (U+1D12B) represents the double flat sign.
Although very uncommon and only used in modern classical music, a triple flat can sometimes be found. It lowers a note three semitones.
The note A flat is shown in musical notation in Figure 1, together with A double flat.
In tuning, flat can also mean "slightly lower in pitch". If two simultaneous notes are slightly out of tune, the lower-pitched one (assuming the higher one is properly pitched) is said to be flat with respect to the other.
The order of the flats in key signature notation B, E, A, D, G, C, and F. A mnemonic for this could be: Birds Eat Angleworms Do Gophers Catch Fish?.
Play an A and an A flat (help·info)
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