256/255, the septendecimal kleisma, diasemisma or 255th subharmonic, is a small 17-limit superparticular comma about 6.8 cents in size. It is the difference between 16/15 (classical diatonic semitone) and 17/16 (large septendecimal semitone, also called as minor diatonic semitone), and forms the amount by which a stack consisting of 15/8 and 17/16 falls short of an octave. It differs from 352/351 (the minthma) by 936/935 – an unnoticeable comma measuring about 1.85 cents.
By tempering it out is defined the diasemismic temperament, which enables the diasemismic chords.
The name diasemisma was named by Xenllium in 2023. It is a contraction of diatonic semitone into a single word; it is unrelated to the diasem scale structure. However, septendecimal kleisma and 255th subharmonic were attested much earlier.