A diatonic interval which comes in only one basic size -- as opposed to imperfect intervals, which come in two sizes and must be either "major" (large) or "minor" (small).
In traditional diatonic music, the perfect intervals are the prime (or unison), the 4th, the 5th, and the octave (8ve), and also the 11th (8ve + 4th), 12th (8ve + 5th), and 15th (double-octave).
If a perfect interval is widened by a semitone, it becomes augmented; if it is narrowed by a semitone, it becomes diminished.
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