) is a small musical interval
defined as the difference between four perfect fifths plus two major thirds (in just intonation
) and three octaves. It can be represented by the ratio 2048:2025 and is about 19.5 cents
(). The use of the name diaschisma for this interval is due to Helmholtz
; earlier Rameau
had called that interval a "diminished comma".
Medieval theorists Boethius and Tinctoris described the diaschisma as one-half of the Pythagorean minor second, or 256/243, which would give an interval of 45 cents.
Tempering out the diaschisma, in the modern meaning of the term, leads to diaschismic temperament. The diaschisma is tempered out in the usual system of 12 equal temperament; in fact, this temperament can be characterized as a 5-limit temperament as the one tempering out both the syntonic comma of 81/80 and the diaschisma. However, it is possible to improve the tuning a good deal over that of 12-et and still temper out the diaschisma; the equal temperaments with 22, 34 and 46 notes all temper it out.