Kleisma

In music theory, the kleisma is an interval important to temperaments of the Bohlen-Pierce scale. It is the difference between six minor thirds and the "tritave" or perfect twelfth (an octave plus a perfect fifth), equal to a frequency ratio of 15625:15552, or approximately 8.1 cents.

The interval was named by Shohé Tanaka, who noted that it was tempered to a unison by 53 equal temperament. Interestingly, it is "not" tempered out, despite its minute size, in 12 equal temperament where six minor thirds are 18 semitones while a perfect twelfth is 19 semitones.