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: Rp (maximal similarity with respect to pitch class), R0 (minimal similarity), R1 (first order maximal similarity), and R2 (second order maximal similarity).

Source: Wikipedia — Similarity relation (music) (CC BY-SA 4.0)