In addition to [[interval size|numerical size]], each interval has a specific quality that distinguishes it from other intervals of the same size but with a different number of half steps. Intervals can be grouped according to the possible qualities they may have: (1) *perfect* intervals and (2) *major–minor* intervals. ## Perfect Intervals The perfect intervals include unisons, fourths, fifths, and octaves and their corresponding compound intervals. If we expand a perfect interval by a half step, it becomes **augmented**. If we contract it by a half step, it becomes **diminished**. ## Major–Minor Intervals The major–minor intervals include seconds, thirds, sixths, and sevenths, and their corresponding compound intervals. A major interval is one half step larger than a minor interval of the same numerical size. If we expand a major interval by a half step, it becomes augmented. If we contract a minor interval by a half step, it becomes diminished. > [!NOTE] Important! > Unisons, fourths, fifths, and octaves are never major or minor. Conversely, seconds, thirds, sixths, and sevenths are never perfect.