47: The 6/4 Chord

 

The 6/4 is much more restricted than the 6, or the completely free root position chord because it creates an instability. Several articles ago, I mentioned that the perfect fourth can be a dissonance in certain situations against the bass, in that it wants to resolve by falling a half step into a major third, despite the fact that, classically, perfect fourths are consonant since they’re perfect.

The 6/4 therefore, as Dr. Perry Holbrook (my high school orchestra director and theory teacher) put it, requires a special license before you can use it, and that license will be granted on a case-by-case basis in only one of 3 situations.

1.       A passing 6/4 is allowed as long as no other rules are broken by including it, and as long as there is movement into and out of it by chords that aren’t 6/4, and the movement out of it resolves anything that needs to be resolved correctly

2.       A pedal 6/4 is allowed as long as you don’t come into the pedal by rising or falling by a tritone (or exit that way), or ever have a sustained tritone against the pedal (a passing tritone is OK), or break any other rules

3.       A cadential 6/4 is allowed if you use a I6/4 to set up a V(7) to set up a I

6/4s naturally have the instability of the fourth built into them—they naturally want the fourth to resolve down to a third (and if it did, you would go, for example, from I6/4 to iii6)—which is why they were used so sparingly, only in these 3 cases and not willy-nilly like root position chords or first inversion chords that act like root position chords, but just sound a little less forceful.

Unless you can (correctly) point to which of these 3 reasons allows you to use it, stay away from the 6/4 chord, or your part-writing will very soon become a mess that you’ll need to untangle—the opposite of how Baroque bass parts in chorales, or continuo parts there and elsewhere, were written. 6/4s used correctly, however, can make an otherwise generic-sounding bassline much more interesting, so use them wisely but have fun!

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