Perfect intervals are intervals that are termed perfect because they have both a high degree of consonance and invert to form other perfect intervals. They are sometimes classified into four types, although the first is not strictly an interval: unison, perfect fourth, perfect fifth, and octave. This contrasts with other intervals, whose inversion changes their quality: that is, the inversion of a major interval is a minor interval and the inversion of an augmented interval is a diminished interval.