The major sixth on drop D guitar

A major sixth is nine semitones, from C to A. Inverted, with the lower note moved up an octave, it becomes the minor third.

Its short name is M6. Counted in letters, it spans six letter names, which is why it is written C to A rather than with another name for the same pitch.

On drop D guitar, the major sixth is two frets up, four frets up and five frets up when you cross to the next string, and nine frets up along one string.

The shape changes from one pair of strings to another, because the strings of this tuning are not evenly spaced. Instead of one shape to learn, there are three of them.

On a treble staff, the two notes are written C and A.

The staff shows the two notes one after the other, then together. C to A spans six letter names, which is what the interval's name counts. What you hear is a distance of nine semitones.

A keyboard makes this distance easy to see, where a fretboard hides it. The upper note is nine keys along from the lower one, counting black and white keys alike.

24 notes are marked across six strings.

This diagram marks both notes of the major sixth, C and A, everywhere they fall on drop D guitar. You can find every pair shown above somewhere on it.

C–A, D♭–B♭, D–B, E♭–C, E–C♯, F–D, F♯–D♯, G–E, A♭–F, A–F♯, B♭–G, B–G♯

The shapes above do not change with the key, which is the reason to learn an interval as a shape. Only the note names change, and this table gives them for every starting note.

One chord types contain a major sixth: Diminished 7th.

These are the chords with a major sixth above their root. Some of them spell that note under another name, but it is the same distance and the same shape on the neck. A chord is built by stacking intervals on a root, and a triad is the simplest case, with two of them.