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: Harmony is the use and study of pitch simultaneity and chords, actual or implied, in music. ... Sesquitertium refers to the improper rational fraction It is a superparticular number. ... In mathematics, the harmonic mean is one of several methods of calculating an average. ... In music, a unison is an interval, the ratio of 1:1 or 0 halfsteps and zero cents. ... The word diapason is used in a number of musical contexts. ...

It is equal to diapason divided by diapente: In harmony, diapente is the ratio 3:2 (sesquialterum) between a pair of frequencies or, equivalently, the ratio 2:3 between a pair of wavelengths. ...

therefore it is the inversion of diapente.

It is the sum of the first 2 reciprocals of triangular numbers: A triangular number is a number that can be arranged in the shape of an equilateral triangle. ...

and it is 1.01010101... in binary.

It is also called perfect fourth. The musical interval of a perfect fourth, often P4, is the relationship between the first note (the root or tonic) and the fourth note (subdominant) in a major scale. ...

See also: unison, diapason, diapente, ditonus, semiditonus, tonus, semitonium. In music, a unison is an interval, the ratio of 1:1 or 0 halfsteps and zero cents. ... The word diapason is used in a number of musical contexts. ... In harmony, diapente is the ratio 3:2 (sesquialterum) between a pair of frequencies or, equivalently, the ratio 2:3 between a pair of wavelengths. ... In harmony, the ditonus (Latin for double tone — plural ditoni) is the ratio 5:4 (sesquiquartum) between a pair of frequencies or, equivalently, the ratio 4:5 between a pair of wavelengths. ... In harmony, the semiditonus is the ratio 6:5 (sesquiquintum) between a pair of frequencies or, equivalently, the ratio 5:6 between a pair of wavelengths. ... In harmony, the tonus is the ratio 9:8 between a pair of frequencies or, equivalently, the ratio 8:9 between a pair of wavelengths. ... In harmony, the semitonium is the ratio 17:16 — or 18:17 — between a pair of frequencies or, equivalently, the ratio 16:17 — or 17:18 — between a pair of wavelengths (or lengths of a monochord). ...