This looks clear as mud at first perusal, but patiently plowing through may pay off. And this also may help.

I've been studying music for fifty years, but I'm still a bit fuzzy as to why the dominant chord is called the "dominant chord." Because it dominates the key? That's what I was told when I asked the question in a music theory class back in 1957. ("Well, okay, if you say so.")

I think it's probably because the dominant seventh (say, a G7) can't be in any key other than its home key (C). It contains G,B,D, and F. The two most closely related keys (keys that share the most notes in the scale) are G and F. The key of G has an F# in it, so the G7 can't be a chord in the key of G. The key of F has a Bb in it, so the G7 can't be in the key of F. The farther you go around the circle of fifths in either direction, the worse it gets. So the G7 points at the key of C as if to say "this is where I belong."

The dominant is always built on the fifth degree of the scale (in the key of C, that would be G). You would use a dominant seventh chord when you want to establish a "drop the other shoe" effect. By piling an F on top of a G triad (G, B, and D), you create a dissonance between the B and the F. The ear wants this dissonance resolved. The F wants to move down to an E and the B wants to move up to a C. It points at the C chord. Once that's done, the tension is relieved. Try playing a G7 and listen carefully to what the chord seems to want to do—or more precisely, what you feel like the chord should do. Then play a C. Does it sound like it's done it? Mission accomplished.

But you don't have to go there. Try playing a G7, then follow it with an F. That's called a "deceptive cadence." But sooner or later, you want to go back to the C.

I'll be back if needed.

Don Firth