Practice

The Major 3rd Interval (M3)

An interval is the distance between two notes, counted in semitones. Each of the thirteen simple intervals has its own page: reference songs that open with it, where it sits on the guitar fretboard, its inversion, and the chords built from it.

The major 3rd spans 4 semitones — the brighter third — the sound of major. Measured up from a major-key tonic it lands on the 3rd, so it is diatonic to every major key. Inverted (the lower note moved up an octave) it becomes the minor 6th.

The classic way to internalize it: sing a song that opens with it — When the Saints Go Marching In (the first leap, "Oh when"); Swing Low, Sweet Chariot (the fall on "Swing low"). You also hear the major 3rd inside chords built on it, like Major, 7, maj7, 6. Then drill it on the ear trainer until you name it without thinking.