Music theory · 6 min read

Intervals explained

An interval is the distance between two notes, named by letters counted and by quality. Here is how to name any interval and hear it without counting.

2026-05-20

Counting the number

To find the number, count letter names from the bottom note to the top, including both ends. From C to E you count C, D, E, which is three letters, so the interval is a third. From C to G you count C, D, E, F, G, five letters, a fifth. From C to the C above, eight letters, an octave.

Inclusive counting trips up almost everyone at first, because it means a note to itself is a first, called a unison, rather than a zero. The consolation is that this convention is centuries old and nobody is going to change it now.

Note that the number ignores sharps and flats entirely. C to E is a third. C to E flat is also a third. C sharp to E is still a third. The letters are the same, so the number is the same. What changes is the second half of the name.

The quality, and why there are two families

Quality tells you the exact size. Intervals fall into two families that behave differently, which is a historical inheritance and not an arbitrary cruelty.

Unisons, fourths, fifths and octaves are called perfect. They are the intervals with the simplest frequency ratios, and the ones that fuse most completely when sounded together. A perfect fifth, seven semitones, is C up to G. A perfect fourth, five semitones, is C up to F. An octave is twelve semitones.

Seconds, thirds, sixths and sevenths come in major and minor versions, one semitone apart. A major third is four semitones, C to E; a minor third is three, C to E flat. A major sixth is nine semitones, C to A; a minor sixth is eight, C to A flat. A major second is two semitones, C to D; a minor second is one, C to D flat, which is the smallest step there is.

Shrink any interval by a further semitone and it becomes diminished. Stretch it by a semitone and it becomes augmented. A perfect fifth C to G, narrowed to C to G flat, is a diminished fifth: six semitones, the interval medieval theorists nicknamed the devil in music, and the one that gives a dominant seventh chord its restlessness.

Hearing intervals without counting

Counting is fine on paper and useless in real time. What you want is recognition, and the standard route is to attach each interval to a tune you already know so the sound arrives with a label.

A minor second is the shark theme from Jaws. A major second is the opening of Happy Birthday. A major third opens When the Saints Go Marching In. A perfect fourth is the first two notes of the wedding march, or of Auld Lang Syne. A perfect fifth is the Star Wars main theme, or the opening of Twinkle Twinkle. A major sixth is the start of the NBC chime, and an octave is Somewhere Over the Rainbow.

Use whichever tunes you actually know rather than a list you have inherited. And practise downwards too, which nearly everyone neglects and which is far harder. A descending minor sixth and a descending perfect fifth are easy to confuse until you have deliberately worked on them.

Intervals as the raw material of chords

Chords are stacked intervals, which is why interval work pays off so quickly. A major triad is a major third with a minor third on top: C to E is four semitones, E to G is three, and the outer distance C to G is a perfect fifth. A minor triad is the same components in the opposite order, minor third then major third: C to E flat, then E flat to G.

A diminished triad stacks two minor thirds, C, E flat, G flat, and its outer interval is that unstable diminished fifth. An augmented triad stacks two major thirds, C, E, G sharp. Four triad types, all describable in one sentence each, once you have intervals.

The same logic runs through everything above it. A dominant seventh chord is a major triad with a minor seventh on top, ten semitones from the root. Learn the intervals and chord construction stops being memorisation.

Intervals on the instrument

The double bass makes intervals physical in a way few instruments do. Its open strings are tuned in perfect fourths, E, A, D and G, five semitones apart, so a fourth is simply the next string across. In the lower positions your hand naturally spans a tone, first finger to fourth; anything larger means a shift. Thumb position, up where the string is short, changes the geometry: the same physical stretch now covers a much wider interval, which is exactly why it exists.

Harmonics turn intervals into audible physics. Touch a bass string lightly at its midpoint and you sound an octave above the open note; touch at a third of the way along and you get an octave plus a fifth. The frequency ratios that define an octave as two to one and a fifth as three to two are right there under the finger.

On the accordion, the chromatic button layout arranges its rows in minor thirds, so a diminished seventh chord is a single straight shape across the board and transposing anything is a matter of moving the shape. In the left hand, the Stradella fundamental row is a chain of perfect fifths: one button in one direction is always a fifth up, one in the other is always a fifth down. Students at A&G Academy often report that the interval finally clicked when they noticed their left hand had been walking in fifths all along.

Frequently asked

Why is C to E a third when it is only two steps?

Because interval numbers count letter names inclusively, including both the starting and ending notes. C, D, E is three letters, so the interval is a third. It is a naming convention rather than a measurement of steps, and the quality half of the name, major or minor, carries the precise size in semitones.

What is the difference between an augmented fourth and a diminished fifth?

In sound, nothing: both span six semitones, and on a keyboard they are the same two keys. The difference is spelling and function. C to F sharp is an augmented fourth and usually resolves outwards; C to G flat is a diminished fifth and usually resolves inwards. Context decides which spelling is correct.

How do I get better at recognising intervals by ear?

Work in short daily sessions rather than long ones, and start with just three intervals until they are reliable before adding more. Sing each one before checking it, since singing forces genuine internal hearing. Include descending intervals from the beginning, because they are harder and are usually postponed until they become a real weakness.


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