Major and minor thirds

A major third is four half steps and a minor third is three, and that single half step changes the mood of a chord.

Two kinds of third

Thirds are everywhere in Western music. Stack two of them and you get a chord; play them one after the other and you get the outline of a melody. A major third spans four half steps, for example C to E. A minor third spans three, for example A to C or E to G. On the staff both look identical, two notes on neighbouring lines or in neighbouring spaces, so the only way to tell them apart from the page is to know which letters are involved and whether there are any sharps or flats.

The white-key thirds

Open Music Theory gives a short list worth memorising: among the white keys, every third is minor except three, C to E, F to A and G to B, which are major. The reason is the half steps E to F and B to C. A third that contains one of them, such as D to F or E to G, is only three half steps. A third that avoids both, such as C to E, is four. Once you know the white-key thirds, accidentals adjust them: D to F is minor, so D to F♯ is major. E to G is minor, so E to G♯ is major.

Thirds make chords

A triad is two thirds stacked on top of each other. C, E and G form a major triad: a major third C to E with a minor third E to G on top. Swap the order, minor third below and major third above, and you get a minor triad, such as A, C, E. People often describe major chords as bright and minor chords as darker or sadder. That is a cultural habit more than a law of nature, but it is strong enough that a single changed note, E to E♭ in a C chord, is heard instantly by almost every listener.

Thirds in melodies

Many traditional tunes open with a third, which makes them handy reference songs. "When the Saints Go Marching In" begins with a rising major third: "Oh when" is C up to E. Brahms' Lullaby, published in 1868, opens with a rising minor third on its first phrase, E to G. The English folk song "Greensleeves" also starts with a minor third, A up to C. Sing one of these to yourself before the ear exercise, then listen for the same size of jump.

On the keyboard you can turn any major third into a minor third by moving the top note one key to the left, or the bottom note one key to the right. Try C to E, then C to E♭; or F to A, then F to A♭. On paper the two notes are still a third apart, because the letters have not changed, but the sound darkens at once. In the ear exercise below, listen for that change of colour rather than trying to measure the distance.