AP® Music Theory · Unit 2 · Topic 2.5
Interval Size: Counting Lines and Spaces
An interval's numeric size comes from counting every letter name from the lower note to the higher note, including both endpoints. Count letters, not half steps, and never start from zero.
Interval size answers one question: how many letter names apart are these two notes, counting both of them? Start on the lower pitch, call it one, then count upward through the musical alphabet, A B C D E F G, until you land on the other note. Whatever number you land on is the interval's numeric size. This is why an interval between a note and itself is called a unison, size 1, not zero, and why going up by one letter name, like C to D, is called a second, not a first.
The mechanism is just inclusive counting. C up to G: C is 1, D is 2, E is 3, F is 4, G is 5, so that's a fifth. E up to D: E 1, F 2, G 3, A 4, B 5, C 6, D 7, a seventh. F down to B: count downward through the alphabet, F 1, E 2, D 3, C 4, B 5, a fifth. Direction never changes the counting method, only which note you start from.
Numeric size is only half the story. C up to E and C up to E flat are both thirds, both size 3, because the letter names C, D, E are the same either way. What separates them is quality: major, minor, perfect, augmented, or diminished, which depends on the actual number of half steps inside that letter-name span. Size tells you the skeleton, quality tells you the exact distance, and the next step in this unit is learning to measure quality precisely.
Key terms
- Interval
- The distance between two pitches, described by a numeric size and a quality.
- Numeric size
- The count of letter names from the lower note to the higher note, counting both endpoints.
- Unison
- An interval of size 1, between a note and itself or its exact duplicate.
- Compound interval
- An interval larger than an octave, whose size is found by adding extra letter names beyond the 8 of the octave.
- Interval quality
- A label such as major, minor, perfect, augmented, or diminished that specifies the exact half-step distance within a given numeric size.
Watch it explained
Check yourself
End of topic.