In
music theory, the
syntonic comma, also known as the
chromatic diesis, the
comma of Didymus, the
Ptolemaic comma, or the
diatonic comma is a small
comma type
interval between two
musical notes, equal to the frequency ratio 81:80 (around 21.51
cents). Two notes that differ by this interval would sound different from each other even to untrained ears, but would be close enough that they would be more likely interpreted as out-of-tune versions of the same note than as different notes. The comma is referred to as a "comma of Didymus" because it is the amount by which Didymus corrected the
Pythagorean major third (81:64, around 407.82 cents) to a
just major third (5:4, around 386.31 cents).