What is an interval in music?

An interval is the distance between two notes. It gets measured two ways at once, which is the only reason it seems complicated.

An interval is the distance between two notes. That is the entire definition, and if it were left there, nobody would find intervals difficult.

This is step three of the music theory path. It assumes you can read notes on a staff and count rhythm — steps one and two.

The difficulty comes from the naming. Every interval is measured two different ways at the same time, and both measurements end up in its name.

The number counts letter names

Start on the lower note and count letter names up to the higher one, including both ends. C to E is C, D, E — three letters, so it is some kind of third. C to G is C, D, E, F, G — five letters, so it is some kind of fifth.

This is why the counting feels wrong at first. C to D is a second, not a first, because you count the note you started on. There is no interval called a first; the same note twice is a unison.

The quality counts semitones

The letter count alone is not enough, because thirds come in two sizes. C to E spans four semitones. C to E flat spans three. Both are thirds by letter count, but they sound completely different — one bright, one dark.

So each interval gets a quality as well as a number. Seconds, thirds, sixths and sevenths are either major or minor, one semitone apart. Fourths, fifths and octaves are called perfect.

Why some intervals are called perfect

The perfect intervals are the ones where the two notes vibrate in the simplest mathematical ratios — two to one for an octave, three to two for a fifth. They sound so consonant that they barely register as two separate notes. They do not come in major and minor versions; instead they can be widened to augmented or narrowed to diminished, which happens far less often.

You do not need the physics. You need to know that fourths, fifths and octaves behave differently from everything else.

Hear the difference first

Before any of the naming, listen to this. Two intervals, one semitone apart, and the entire emotional distance between bright and dark in Western music.

The full list

Twelve semitones, twelve intervals, then it repeats an octave higher. Each keyboard shows the two notes — grey is the note you start on, red is where you land. Press both to hear them together, or apart to hear them one after the other.

Why any of this matters

Intervals are not a topic you study and then move past. They are the material everything else is made of.

Chords are stacked intervals. A major chord is a major third with a minor third on top of it. Change the lower third to a minor third and you have a minor chord. That is the only difference between the two.

Scales are patterns of intervals. A major scale is a fixed sequence of whole and half steps — two, two, one, two, two, two, one. Start that pattern on any note and you get that note’s major scale. The pattern is the scale; the starting note is just where you put it.

Keys relate to each other by interval. The circle of fifths is called that because each step around it is a perfect fifth. It is one interval, applied twelve times.

How to actually learn them

Do not memorise the table. Learn to hear three of them first: the octave, the perfect fifth, and the major third. Those three are the skeleton of nearly all Western harmony — and they are what the chord finder stacks to build every chord it draws, and once you can pick them out, the others sit in relation to them.

Play two notes on your instrument, name the interval, then check. A few minutes a day beats an hour once a week, because this is recognition rather than understanding — and recognition only comes from repetition.

Next: chords, which are what happens when you stack these on top of each other.