Section 1Introduction
A major third is four semitones — the interval that defines the major triad and gives major keys their characteristically bright sound. It is one of the most consonant intervals after the perfect octave and fifth, and its 5:4 frequency ratio is responsible for the "warm" character of just-intonation tuning.
Section 2How to Find It on the Keyboard
Find any major third in two simple steps. The number tells you the letter. The semitones tell you the accidental.
- Start on any root note. Count 3 letter names (including the root) up the musical alphabet — that gives you the top letter.
- Now count exactly 4 semitones from the root. If the natural top letter is too high or too low, sharpen or flatten it to land on the right pitch.
- Use the explorer above to check yourself in all 12 keys. The two highlighted notes are the M3 from that root.
Quick check: from C, the M3 lands on E. From G, it lands on B. From E♭, it lands on G.
Section 3Hear It — Song Associations for Ear Training
The fastest way to internalise the major third is to associate it with a tune you already know. Sing the first two notes of any of these and you have the interval.
Section 4The Interval in Chords
Every chord is a stack of intervals. Here is where the major third shows up in common harmony.
| Chord | Name | How M3 Appears |
|---|---|---|
| C | Major triad | Root + major 3rd + perfect 5th |
| C+ | Augmented triad | Two stacked major thirds |
| Cmaj7 | Major 7th chord | Major triad with a major 7th on top |
Section 5Inversion: Flip It Upside Down
When you move the bottom note up an octave (or the top note down an octave), the interval inverts. Two simple rules govern interval inversions:
- Numbers sum to 9. A 2nd inverts to a 7th, a 3rd inverts to a 6th, a 4th inverts to a 5th, and so on (1 + 8 = 9 for unison/octave).
- Quality flips. Major ↔ minor, augmented ↔ diminished, perfect stays perfect.
Section 6Compound Form
A compound interval is the same interval with an extra octave added on top. The character stays the same but the two notes are spread further apart. The compound form of the major third is the Major Tenth (M10) — 16 semitones in total.
Why it matters: chord extensions like 9ths, 11ths, and 13ths are compound intervals stacked above the basic triad. Move it an octave up and you get the 10th — a bigger, more open sound that's common in piano voicings.
Section 7Enharmonic Equivalents
Two intervals are enharmonic when they sound the same but are spelled differently. Same physical pitches, different musical meaning.
- Diminished Fourth (d4) — four semitones, e.g. C–F♭
On the keyboard, an enharmonic pair sounds identical. On paper, the spelling tells you which scale or chord the note belongs to — and that changes how it functions in the music.
Section 8Frequently Asked Questions
Why does a major third sound happy?
The 5:4 frequency ratio produces a clean, bright overtone pattern. Combined with cultural conditioning around major-key music, it reads as "uplifting" to most Western listeners.
What is the inversion of a major third?
A minor sixth. M3 + m6 = 9, major flips to minor.
How do major and minor thirds combine?
A major third on the bottom + minor third on top makes a major triad. Minor + major makes a minor triad. Major + major makes augmented; minor + minor makes diminished.
How do I find a major third quickly?
From any note, count four half steps up. A major third above C is E; above F is A; above A is C♯.
Is a major third in every major scale?
Yes — every major scale opens with a major third between the 1st and 3rd degrees.