Similarity relation (music)

In music, a similarity relation or pitch-class similarity is a comparison between sets of the same cardinality (two sets containing the same number of pitch classes), based upon shared pitch class and/or interval class content.

Allen Forte originally designated four types: R_p (maximal similarity with respect to pitch class), R₀ (minimal similarity), R₁ (first order maximal similarity), and _R2 (second order maximal similarity). In R_p one pitch class is different, in R₀ all are different, and in R₁ and R₂ four interval classes are the same.