I covered most chord structures in this post from last year.

Two things I didn’t cover were mixed-interval chords and whole-tone chords.

We talked about whole-tone chords when we looked at scales in the last chapter. Dominant seventh chords with a lowered fifth (enharmonically equivalent to a French augmented sixth chord) and raised fifth are both possible. The lowered fifth is more common, especially in French music.

Mixed-interval chords occur more often in atonal music, and you should resist the urge to quickly label a chord mixed interval without first considering your other options. In most of the scale-oriented music that we are looking at right now, chords will be built of similar intervals (tertian variety, quartel, quintal, seconds) or from a scale formation being used (whole-tone).