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4.2.9Types of Intervals: The Complementary Intervals
Any two intervals that add up to an octave (which consists of 12 semitones) are called complementary intervals (Table 17).
TABLE 17 The Complementary Intervals
| Minor 2nd (1 semitone) | + Major 7th (11 semitones) | = Octave |
| Major 2nd (2 semitones) | + Minor 7th (10 semitones) | = Octave |
| Minor 3rd (3 semitones) | + Major 6th (9 semitones) | = Octave |
| Major 3rd (4 semitones) | + Minor 6th (8 semitones) | = Octave |
| Perfect 4th (5 semitones) | + Perfect 5th (7 semitones) | = Octave |
A few “rules” of complementary intervals:
- complement of any minor interval is a major interval. And vice-versa.
- The only two “perfect” intervals—perfect fourth and perfect fifth— complement each other (wouldn’t you know it).
- There’s no complement for the diabolical tritone (6 semitones).
Complementary intervals are important in understanding chord changes or chord progressions, the subject of Chapter 6.