In harmony, the diatessaron is a ratio of 4:3 (sesquitertium) between a pair of frequencies or, equivalently, a ratio of 3:4 between a pair of wavelengths. It is the harmonic mean of unison and diapason:

It is equal to diapason divided by diapente:

therefore it is the inversion of diapente.

It is the sum of the first 2 reciprocals of triangular numbers:

and it is 1.01010101... in binary.

It is also called perfect fourth.

See also: unison, diapason, diapente, ditonus, semiditonus, tonus, semitonium.