Syntonic comma

The syntonic comma, also known as the comma of Didymus, is a small interval between two musical notes, equal to the frequency ratio 81:80, or around 21.51 cents.

It is the difference between four justly tuned perfect fifths, and two octaves plus a justly tuned major third. A just perfect fifth has its notes in the frequency ratio 3:2, which is equal to 702.96 cents, and four of them are equal to 2807.82 cents. A just major third has its notes in the frequency ratio 5:4, which is equal to 386.31 cents, and one of them plus two octaves is equal to 2786.31 cents. The difference between these is 21.51, a syntonic comma.

This difference is significant because on a piano keyboard, four fifths is equal to two octaves plus a major third. Starting from a C, both combinations of intervals will end up at E. The fact that using justly tuned intervals yields two slightly different notes means that it is not possible to tune perfect fifths and major thirds perfectly in any system of musical tuning. Tuning systems must instead make a compromise between these two intervals. Pythagorean tuning tunes the fifths as exact 3:2s, but uses the relatively complex ratio of 81:64 for major thirds. Quarter comma meantone, on the other hand, uses exact 5:4s for major thirds, but flattens each of the fifths by a quarter of a syntonic comma. Other systems use different compromises.