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<sub>p</sub> (maximal similarity with respect to pitch class), R<sub>0</sub> (minimal similarity), R<sub>1</sub> (first order maximal similarity), and R<sub>2</sub> (second order maximal similarity). In R<sub>p</sub> one pitch class is different, in R<sub>0</sub> all are different, and in R<sub>1</sub> and R<sub>2</sub> four interval classes are the same.
Rp is defined for sets S<sub>1</sub> and S<sub>2</sub> of cardinal number n and S<sub>3</sub> of cardinal number n-1 as:
Meaning that S<sub>1</sub> and S<sub>2</sub> each have all the pitch-classes of S<sub>3</sub> (transposed or inverted), plus one.