The Interval of a unison, 5th, octave or any of their compounds (12th, 15th, etc.), when neither augmented nor diminished. The term is contrasted in much medieval polyphonic theory with ‘imperfect consonance’, a simple or compound 3rd or 6th (see Counterpoint, §2). The diatonic Fourth, although usually defined as a dissonance in that context, has since been considered a Consonance when understood as the inversion of the perfect fifth; as a result, the term ‘perfect interval’ is generally taken to include the ‘perfect 4th’, and its compounds in addition to the medieval perfect consonances.
Julian Rushton