I want to enlighten you about the proper use of second inversion triads. That’s right, folks: the much-talked-about six-four chord! Let’s get one thing straight right off the bat: it’s DISSONANT, and must be treated as such. Once you get that simple fact through your heads, then you can begin to understand why it has only certain, limited legitimate uses.
A tonic six-four chord, for example, is just a dominant chord with two appoggiature over the third and fifth degrees above the bass; this is why most six-four chords RESOLVE to a root-position triad (a five-three chord), while retaining the same note in the bass. DUH! This usage applies to both the APPOGGIATURA six-four and the CADENTIAL six-four. (Look it up; I wouldn’t lie about it, and I certainly couldn’t make this stuff up!)
In case you were wondering (and you were), the other three sanctioned uses of this chord inversion are:
- the PASSING six-four chord,
- the AUXILIARY (or NEIGHBOR) six-four,
- and the six-four that results from ARPEGGIATION in the bass.
Please exercise prudent judgment, and from now on, don’t let me catch you guys misusing this fine harmony! Any questions?
P.S. In four voices, it is the BASS that is usually doubled in a second-inversion chord, for the simple reason that one shouldn’t double a dissonant note. DUH!