syntonic comma

Noun

 * 1)  A small comma-type interval between two musical notes, equal to the frequency ratio 81:80 (= 1.0125) or around 21.51 cents. It often manifests itself as the difference between the Pythagorean major third of 81:64 and the classic major third of 5:4, between the Pythagorean minor third of 32:27 and the classic minor third of 6:5, and between the "major whole tone" of 9:8 and "minor whole tone" of 10:9.