All of the intervals mentioned below are presumed to be tuned in just intonation.
There are many commas, notably :
Commas are frequently used in the description of musical temperaments , where they describe distinctions between musical intervals that are eliminated by that tuning system. A comma can be viewed as the distance between two musical intervals. When a given comma is tempered out in a tuning system, the ability to distinguish between those two intervals in that tuning is eliminated. For example, the difference between the diatonic semitone and chromatic semitone is called the diesis. The widely used twelve-tone equal temperament tempers out the diesis, and thus does not distinguish between the two different types of semitones. On the other hand, nineteen-tone equal temperament does not temper out this comma, and thus it distinguishes between the two semitones.
There are also several intervals called commas, which are not technically commas because they are not rational fractions like those above, but are irrational approximations of them. These include the Holdrian comma and Mercator's comma.