A Dictionary of Music and Musicians/Flat
FLAT. A term employed in the sense of lowering; an artist sings or plays flat when his notes are below the right pitch. B flat is a semitone lower than B, E flat than E, and so on; to 'flatten' (baisser) a sound or an instrument is to make it lower than before, just as to 'sharpen' it is to raise it. The sign used to denote this flattening in music is ♭, called a flat—Fr. bémol; Ital. Bemolle; Germ. Be. It has been already shown under Accidentals and B (p. 19a and 107) how the signs of the flat (♭) and natural (♮) were derived from two forms of the letter b. A double flat is a descent of two semitones, and is marked by ♭♭. The flat of a note is not the same pitch (does not give the same number of vibrations) as the sharp of the note a tone below it, though on a keyed instrument the two are represented by the same black key; nor are B and E the same as C♭ and F♭—and so on. This will be explained under Interval.
In German musical nomenclature the notes are flattened by adding es to the letter, as Es, Des, Ges, etc., A flat is As, and B flat B, though Hes has been used. Double flats are Deses, etc. The ♭ and ♯ in German literature were formerly used to express minor and major, as G♭ for G minor, D♯ for D major, and even E♭ for E minor, and As♯ for A flat major. (See the earlier Indexes of the Allgemeine musikalische Zeitung for frequent instances of this strange usage.) Such ambiguities are now avoided by the use of the words dur and moll for major and minor.