Two notes, and the space between them. That space is an interval — and it's the thing your ear actually reacts to. Melodies are intervals in a row; chords are intervals stacked up. Learn to measure the gap and you can name any distance in music.
An interval is a distance
An interval is simply how far apart two notes are. C up to G is an interval. E up to G is a (smaller) interval. That's the whole idea — but there are two different rulers for measuring it, and the trick to intervals is never mixing them up.
The confusion: intervals get measured two ways at once. A number (2nd, 3rd, 5th…) counts letter names. A quality (major, minor, perfect) counts the actual half steps. You need both to name an interval — like "major 3rd" — and each comes from a different ruler.
| Ruler | How you count | C up to G |
|---|---|---|
| Number (2nd, 3rd, 4th…) | count the letter names, including both ends | C·D·E·F·G = 5 letters → a 5th |
| Quality (semitones) | count the half steps for real | 7 half steps |
The number is just a letter count. C to G spans five letters, so it's some kind of 5th — no matter what the actual distance is. The quality fills in the rest.
Why the same number can be two sizes
Remember from Notes that the white keys are unevenly spaced — only E–F and B–C are half steps, everything else is a whole step. That unevenness means counting the same number of letters can cover different distances.
Watch two different "3rds" (three letters each) come out different sizes:
Both are "3rds" — three letters wide. But E→G squeezes through the E–F half step, so it's one half step narrower. The wider one is a major 3rd (4 half steps); the narrower is a minor 3rd (3 half steps). Same number, different quality.
Perfect vs. major/minor
Intervals split into two families based on how they behave:
- Perfect intervals — the 4th, 5th, and octave (and the unison). These come in only one natural size, so they don't get a major/minor label. They sound hollow and stable — a power chord is just a perfect 5th.
- Major/minor intervals — the 2nd, 3rd, 6th, and 7th. These come in a wider and a narrower size, so each is either major (wider) or minor (narrower, one half step smaller).
You don't have to memorize the half-step counts — count letters for the number, then check if it's the wide or narrow version. Here are the everyday ones, all measured up from C:
| Interval | Half steps | From C |
|---|---|---|
| Half step (minor 2nd) | 1 | C → D♭ |
| Whole step (major 2nd) | 2 | C → D |
| Minor 3rd | 3 | C → E♭ |
| Major 3rd | 4 | C → E |
| Perfect 4th | 5 | C → F |
| Perfect 5th | 7 | C → G |
| Octave | 12 | C → C |
Push past the edge: augmented and diminished
Stretch a perfect or major interval a half step wider and it becomes augmented; squeeze it a half step narrower and it becomes diminished.
The famous case: F up to B is a 4th (F·G·A·B, four letters) — but because neither E–F nor B–C falls inside it, it's all whole steps: 6 half steps, one wider than a normal perfect 4th (5). That's an augmented 4th. It splits the octave exactly in half, so it has a nickname — the tritone — and it's the tense, unsettled sound you hear in suspense music.
The same 6-half-step gap is also a diminished 5th (a perfect 5th squeezed one half step). Same distance, two names — which one you use depends on the letters you counted from.
The 3rd is the interval chords are made of
One interval matters more than the rest: the 3rd. Stack a note, skip a letter, land on the next — that's a 3rd — then do it again, and you've built a chord. Whether that first 3rd is major or minor is exactly what makes a chord sound bright or dark. Everything in Chords is thirds stacked up.
On guitar, one fret = one half step
Guitar makes intervals visible: each fret is a half step, so any interval is just a fret distance on one string.
| Interval | Half steps | Same string |
|---|---|---|
| Minor 3rd | 3 | 3 frets up |
| Major 3rd | 4 | 4 frets up |
| Perfect 4th | 5 | 5 frets up |
| Perfect 5th | 7 | 7 frets up |
This is also the secret behind standard tuning: most neighboring strings are a perfect 4th (5 frets) apart — E→A, A→D, D→G, B→e. Only G→B is a major 3rd (4 frets), which is why one chord shape shifts a fret right there. Every shape you learn on the neck sits on top of that "mostly 4ths" grid.
In one line: an interval is the distance between two notes, named with two rulers at once — a number from counting letters (3rd, 5th…) and a quality from counting half steps (major/minor/perfect); the 3rd is the one chords are built from.