Comma of Didymus

The musical interval by which four fifths exceed a seventeenth (i.e., two octaves and a major third),

$${({\textstyle{3\over 2}})^4\over 2^2({\textstyle{5\over 4}})} = {3^4\over 2^4\cdot 5} = {81\over 80} = 1.0125,$$

also called a Syntonic Comma.

See also Comma of Pythagoras, Diesis, Schisma