Probably one of the most common six-four chord is the cadential 6/4. There are, however, several other six-four chords which can broadly be divided into two groups: accented – occur on a stronger beat or part of a beat to the chord of resolution – and, unaccented – occur on a weaker beat or part of a beat to the chord of resolution. Although these groupings reflect the metric position of these chords, as we shall see, many six-four chords can be used either accented or unaccented.
The cadential six-four is an accented six-four chord as it typically occurs on a metrically stronger beat to its chord of resolution. The other common accented six-four chord resembles the cadential six-four in its structure and metric position. Unlike the cadential six-four, however, the accented six-four chord is not cadential and can be used on several scale degrees; however, it typically avoids a resolution to the root position diminished chord (see below).
Looking at the following, we can see that both the cadential six-four and the accented six-four delay the arrival of the chord of resolution; the cadential six-four delays the arrival of the dominant or dominant seventh chord while the accented six-four delays the arrival of another chord.
Notice, with both chord-types the labelling reflects the decorative nature of the vertical ‘six-four’ intervals: it is the 5/3 chord which is the true harmony, the 6/4 is merely delaying the 5/3’s arrival.
As noted previously, a resolution to a diminished chord is typically avoided. When using accented six-four chords on the leading note of a major key, or sharpened leading note of a minor key, therefore, the 6/4 intervals typically move onto a first inversion dominant seventh chord rather than to a root position diminished triad.
Frequently, a six-four chord is used after its chord of resolution.
Here, the 6/4 intervals still decorate the 5/3 chord, albeit, after the chord of resolution – the labelling therefore still indicates the underlying progression from tonic to dominant and not three separate chords.
The next type of six-four chord is the neighbouring six-four chord, which occurs above the same bass note as the chord which precedes and follows.
This six-four chord can be used in both accented and unaccented positions, although the unaccented position is more common. Again, the 6/4 intervals are decorating the 5/3 chord; the neighbouring six-four chord prolongs 5/3 harmony.
Like the accented six-four chord discussed previously, the neighbouring six-four typically does not decorate a diminished chord.
The next type of six-four chord is the passing six-four. Like the neighbouring six-four chord the passing six-four can be used in either accented or unaccented positions, although unaccented is more frequent. Also, like neighbouring six-four chords the passing six-four prolongs other harmony. It is often found prolonging tonic, supertonic or subdominant harmony; note the voice exchange between the treble and bass.
Passing six-four chords also often prolong seventh chords by moving between two of the seventh chord’s inversions. Once again, a voice exchange is used between the treble and bass
Passing six-four chords can also be used between two different chords. A very common usage is to insert a passing six-four between the subdominant and the supertonic seventh when both chords are in first inversion. Here, the outer parts move in parallel sixths
Passing six-four chords are also frequently found above a sustained bass. In these instances the context, duration of the 6/4 intervals and metric placement should clarify whether it is a true passing six-four chord or simply passing notes moving in thirds or sixths. In either case, the 6/4 intervals are still decorating the 5/3 chord.
Consider the following: in this example the chord on beat two can be heard as a passing six-four as it lasts for a full beat and passes to the next chord
in this example, the A and F notes are best described as passing notes moving in sixths, as they last for only part of the beat while tonic harmony sounds for the first two beats.