An interval is the distance in pitch between two notes. Name it with a number and a quality, a major third, a perfect fifth, a minor seventh, and you have fixed a distance you can hear and reuse in any key. Intervals are the pieces scales and chords are built from, so once they are clear the rest of the theory has something to stand on.
Plain theory, no instrument required. It works the same on a piano, a guitar, or in a piano roll, in any key and any style.
An interval is the distance in pitch between two notes, measured in semitones, the half steps that are the smallest move in Western music. Count the semitones between the lower note and the higher one and you have measured the interval.
On a piano a semitone is the step from any key to the very next one, black or white, with nothing in between. Two semitones make a tone, also called a whole step. So every interval is really just a count of semitones: from C up to the E above it is four semitones, and from C up to the G above it is seven.
Two notes at the very same pitch are a unison, zero semitones apart, and that is where the counting starts. The two notes of an interval can sound at the same time or one after the other; either way the distance between them, and so the interval, is the same.
Every interval has a two part name: a number and a quality, such as a major third, a perfect fifth or a minor seventh. The number gives the rough size and the quality fixes the exact one.
The number comes from counting letter names from the lower note up to the higher one, counting both ends. C up to E is a third, because C, D, E is three note names; C up to G is a fifth, because C, D, E, F, G is five. The quality then pins down the precise width, because a plain number like "third" can be one of two sizes. Seconds, thirds, sixths and sevenths each come in a major and a minor form a semitone apart, the major being the wider of the two. Unisons, fourths, fifths and octaves are instead called perfect.
Here are the common intervals measured up from a root, with their semitone counts and an example starting on C.
| Interval | Semitones | From C |
|---|---|---|
| Minor second | 1 | C to Db |
| Major second | 2 | C to D |
| Minor third | 3 | C to Eb |
| Major third | 4 | C to E |
| Perfect fourth | 5 | C to F |
| Tritone | 6 | C to F# |
| Perfect fifth | 7 | C to G |
| Minor sixth | 8 | C to Ab |
| Major sixth | 9 | C to A |
| Minor seventh | 10 | C to Bb |
| Major seventh | 11 | C to B |
| Octave | 12 | C to C |
The tritone at six semitones sits exactly halfway between the perfect fourth and the perfect fifth, and it is the one interval on the list with two names: an augmented fourth or a diminished fifth, depending on how it is spelled. Augmented just means a semitone wider than usual and diminished a semitone narrower, and in principle any interval can be stretched or squeezed that way. The octave closes the loop at twelve semitones, landing on the same note name again an octave higher, which is why intervals wider than an octave are often read as an octave plus a smaller one.
A melodic interval is two notes played one after the other; a harmonic interval is the same two notes played together. The distance and the name do not change, only the timing.
Melodic intervals are what you hear moving through a tune. Each step from one note to the next is an interval, and it can rise or fall, so a melody is really a run of melodic intervals laid end to end. Harmonic intervals are what you hear when two notes sound at once. They are the raw material of harmony, and stacking a few of them is how you get a chord.
The same pair of notes shows both. Play C and then G, one after the other, and you have a melodic perfect fifth; hold C and G together and you have a harmonic perfect fifth. Seven semitones apart either way, and a perfect fifth either way. That is why learning intervals pays off twice: it helps you read the shape of a melody and the make up of a chord with the same set of names.
Intervals are the building blocks of scales and chords, so much of the rest of music theory is really a pattern of intervals under a different name. Get comfortable with them and scales, chords and melodies all start to look like the same few pieces arranged in different ways.
A scale is a fixed pattern of intervals climbing from a starting note, a set order of tones and semitones, which is why a scale keeps its character wherever you begin it. Our guide to scales and keys walks through those patterns and the keys they build.
A chord is built by stacking intervals. Take a root, add a major third above it, then a minor third above that, and you have a major triad. From C that is C, E and G, where C to E is a major third of four semitones, E to G is a minor third of three, and the outer C to G is the perfect fifth of seven. Our guide to what a chord is takes those stacked intervals into the full set of chords.
And because an interval is a fixed distance, a phrase or a chord shape holds its character when you move it to another key. You are shifting every note by the same interval, so the pattern of distances inside it stays intact. Thinking in intervals, rather than in single notes, is a large part of what makes music easy to move around and to hear.
We built ARGISH to make this quick to hear. It is a self-playing chord synth: set a key and it builds chords in that key, so instead of counting semitones on a page you can listen to how intervals stack up into harmony, a major third and a minor third turning into a full chord right under your ear.
It runs free in your browser to try, and installs as AU, VST3, AAX or a standalone app when you want it inside your own sessions.
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