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.
C up to E is a major third. Count the letter names and you get three — C, D, E. Count the semitones and you get four. Both numbers are part of the name, and that is the whole reason intervals feel confusing at first.
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.
The full list
Twelve semitones, twelve intervals, then it repeats an octave higher. This is the whole system:
Unison
The same note.
Minor 2nd
The smallest step there is — one fret, or one key including the black ones.
Major 2nd
A whole step. Two adjacent white keys with a black key between them.
Minor 3rd
The interval that makes a chord sound minor.
Major 3rd
The interval that makes a chord sound major.
Perfect 4th
The gap between most adjacent guitar strings.
Tritone
Exactly half an octave. Unstable, and wants to resolve.
Perfect 5th
The most stable interval after the octave. The step the circle of fifths is built on.
Minor 6th
Major 6th
Minor 7th
The note that turns a major chord into a dominant seventh.
Major 7th
One semitone below the octave. Tense, and characteristic of jazz.
Octave
The same letter name again, higher. Where the pattern restarts.
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 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.