(Ger. Quartsextakkord).
A three-note chord consisting of a bass note with a 6th and 4th above it. In thoroughbass it is indicated by the figure ‘6’ placed above the figure ‘4’. In terms of fundamental bass theory a 6-4 chord is the second inversion of a major or minor triad. In any strict harmonic context it is unstable and, unless used in a transitory context (as a ‘passing 6-4’, Ger. Durchgangsquartsextakkord), must be resolved to a consonant triad. Most often this is achieved by the conjunct descent of both upper notes (i.e. 6-4 resolving to 5-3); in such cases the bass note is often the dominant (‘dominant 6-4’ or ‘cadential 6-4’) and the 6-4 itself functionally a dominant, such that resolution of the upper notes either completes an Imperfect cadence or precedes resolution through a Perfect cadence.
See also Harmony, §2(ii), and Inversion.