The intervals in the major scale · From Zero to Mozart

The quality of an interval says how wide it is, in half steps. There are two families:

2nds, 3rds, 6ths, 7ths come as minor or major — minor being one half step narrower than major.
unisons, 4ths, 5ths, octaves come as perfect — never "major" or "minor".

The quickest way to get your bearings is to measure everything from a major scale's tonic.

Every interval up from the tonic

64 bpm

Every note of D major, measured as an interval up from D. Major 2nd, major 3rd, perfect 4th, perfect 5th, major 6th, major 7th, perfect octave. All perfect and all major — no minor anything.

From ^1 to…	Interval	Half steps
^2	Major 2nd	2
^3	Major 3rd	4
^4	Perfect 4th	5
^5	Perfect 5th	7
^6	Major 6th	9
^7	Major 7th	11
^8	Perfect octave	12

If you build up from a tonic and land on a minor interval, you've left the major scale. That's a genuinely useful check.

Why some are "perfect"

The unison, octave, 5th and 4th are the intervals that sit at the bottom of the overtone series — the simplest, most blended sounds a vibrating string makes (you'll meet this properly in The harmonic series). They're also the same in major and minor keys: C up to G is a perfect 5th whether you're in C major or C minor. Thirds and sixths change with the mode, so they get labelled "major" or "minor"; these four don't change, so they're just "perfect."

Working out any interval

You won't always be measuring from a tonic. To name any interval — say D up to F:

Number: D–E–F, three letters → a 3rd.
Quality: is F in D major? D major's 3rd is F**♯**. Plain F is a half step lower — so it's been narrowed. A narrowed major 3rd is a minor 3rd.

60 bpm

D up to F♯ is a major 3rd (4 half steps). D up to F natural is a minor 3rd (3 half steps) — the same number, one half step narrower. On the violin, the 2nd finger just slides back.

Turning an interval upside down

Move the bottom note up an octave (or the top note down) and you invert the interval. Two rules:

Rule of 9: the number and its inversion add up to 9. A 3rd inverts to a 6th (3 + 6 = 9); a 2nd to a 7th; a 4th to a 5th.
Quality flips: major ↔ minor, augmented ↔ diminished, perfect stays perfect.

So a major 3rd (C up to E) turned over becomes a minor 6th (E up to C) — the same two notes, seen from the other end. That's why a melody and a harmony line can be described as "a 3rd apart" or "a 6th apart" and mean the same relationship.

Try it

Interval quality

The usual mistake

Assuming a "3rd" is always major or a "6th" always sounds sweet. The number is only half the name. C–E and C–E♭ are both 3rds, but one is bright and one is dark — and which one a piece uses is often the whole point.