A split sharp is a kind of key found in some early keyboard instruments, such as the harpsichord, clavichord, or organ. It is a musical key divided in two, with separately depressible front and back sections, each sounding its own pitch. The particular keys that were split were those that play (C♯, D♯, F♯, G♯, A♯) on the standard musical keyboard. (On a modern piano these are the "black keys," though early instruments often colored them differently.)
Split sharps served two distinct purposes. First, in the broken octave, they allowed an instrument to include deep bass notes while retaining a short, compact keyboard. For details, see short octave.
Second, in older music, tuning was generally not done by equal temperament, which treats note pairs such as A# and B♭ as the same pitch. Instead, they were assigned slightly different pitches on enharmonic keyboards (particularly in "meantone temperament"). This allowed certain musical intervals, such as the major third, to sound closer to their ideal just value, hence more closely tuned to Just intonation.[1]
In modern usage, split sharps are usually the method of choice for custom keyboards that play 19 equal temperament, which, like meantone, uses different pitches for sharps and flats that are enharmonic in the standard 12 tone.[2]