What an interval is
Measure an interval in two ways, by letter names and by semitones.
Almost everything in theory is built from one idea: the distance between two notes. That distance is called an interval, and you can measure it in two different ways.
What an interval is
An interval is the distance in pitch between two notes. If you play one note after the other, it is a melodic interval; if you play both together, it is a harmonic interval. An interval can go up (ascending) or down (descending). It can start on any note, not only on the first note of a scale: E→B is an interval too.
Every interval has a number and a quality. This lesson is about the number; the quality comes in lesson 4.2.
The number: count the letters
The interval number counts letter names, including both ends:
- C→E covers C D E: a 3rd;
- C→G covers C D E F G: a 5th;
- C→C an octave higher is an octave (an 8th);
- C→C on the same pitch is a unison (a 1st).
Because both ends are counted, two 3rds stacked make a 5th, not a 6th: C–E plus E–G is C–G.
The size: count the semitones
The semitone count says exactly how big the interval is. On the bass, that is the number of frets. C→E is 4 semitones; C→E♭ is 3. Both are 3rds, because the letters are the same, but they are different intervals. The quality, in the next lesson, names that difference.
On the A string, from C (fret 3), play the note a 3rd above, then a 5th above. Count the letters out loud, then the frets.