A major sixth is the chord you get when you take a major triad and add the sixth note of the major scale. They're notated C6.

For example, an F major triad is F A C. The sixth note of the F major scale is D, so the F6 chord consists of F A C D.

Compare with the minor sixth: the only difference is the underlying triad.

Y'know, if you log in, you can write something here, or contact authors directly on the site. Create a New User if you don't already have an account.