A small pitch interval of fundamental importance to temperament and tuning. There are two types of comma. The ‘syntonic comma’ (also called ‘comma of Didymus’ or ‘Ptolemaic comma’) is the difference between a just major 3rd and four just perfect 5ths less two octaves, which is 21.51 cents. The ‘Pythagorean comma’ (or ‘ditonic comma’) is 23.46 cents, being the difference between twelve 5ths and seven octaves. For practical tuning purposes, the difference between the two types of comma is often ignored and the comma is taken to equal 24 cents.
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