The flat 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 semitone lower, or in modern tuning exactly 100 cents.

[1][2] In traditional and modern microtonal temperaments the size of sharps or flats (chromatic semitones) is normally smaller than the size of the diatonic semitones found between E and F or B and C. In those tuning systems, the size of the shift made by the ♭ symbol usually conforms to the smaller-sized lowering of pitch;[a] however, for some tuning systems it may instead be replaced by a different symbol for raising and lowering pitch, depending on the author's preference and the intricacy of any microtuning involved.

)[4] The symbol of a quadruple flat (),[d][7] or beyond could be used, but would be extremely rare in ordinary temperament.

The Unicode character ♭ (U+266D) can be found in the block Miscellaneous Symbols; its HTML entity is ♭.