As you probably know, other than simple folk tunes, when you play more jazzy songs, or pop, you usually don't use simple triads.
Example: The common progression II-V-I in a folk tune might be Dm - G - C. (Or in folk songs you might use a IV-V-1, like F - G - C.)
But in jazz, you'd normally play II-V-I in seventh chords, like Dm7, G7, CMaj7. (We'll leave pop and blues to the side for now.)
One of the interesting things about this little progression is how urgently that G7 chords yearns to resolve to the C chord. The reason it yearns so hard is that there are two notes in the C-major scale that are super-dissonant against the C-tonic note.
One is B, a half-step below C.
The other is Gb, a flatted-fifth above C.
The reason these two notes are so dissonant against the C-note is that their harmonics do not align with the harmonics of the note C at all.
The G-note, the normal 'Perfect' fifth, has harmonics which align wonderfully with the C-note. And of course the C-note's harmonics align wonderfully with the C-note: they're identical, for it's the same note.
But the B-note is just *off* the C-note, and the flatted-fifth (Gb) is just *off* the G-note. And because of this misalignment of harmonics, the B and the Gb are very dissonant.
When we play the G7 chord, a somewhat similar thing happens. The F-note inside the G7 chord has a similar, bad-harmonic-alignment with the B-note inside the G7 chord. Jarring.
And when we move to the CMaj7 chord, then the F-note resolves to the E-note a half-step down (the third of the CMaj7) and the B-note resolves up to the C-note (the root of the CMaj7), and this all sounds very happy.
How to Use this Knowledge
When playing strictly inside the diatonic key of C, there is only two chords which have this hard-yearning quality: G7 and also the Bm7b5 chord (which is not used nearly as often, and we'll ignore it here).
But you have the freedom to step *out* of the diatonic notes anytime you wish, and here's a way to step out all the time, and make your music drive forward all the faster ...
Let us say that you are about to play a D-chord. Any D-chord.
Let us say you want to make that D-chord, when it arrives, seem really inevitable, and like we'd been really yearning for that D-chord.
Simple. Find the note a fifth above the D-note -- that will be A, and play an A7. And then pretty much any D-chord following will sound like a resolution from that A7.
That A7 is called a 'Secondary Dominant.' It's a dominant chord, by its form, though it's not the true dominant chord that occurs naturally in the diatonic key, so we call it 'Secondary.'
Now given that the II-V-I happens over and over and over in jazz and other forms of music, and given that this and other progressions are usually going down by fifths, you can *insert* these 'Secondary Dominants' to drive around the progression with more driving force.
An Example:
The progression is VIm7 - IIm7 - V7 - IMaj7. In the key of C that would be Am7, Dm7, G7, and CMaj7.
Now insert secondary dominants like this:
Am7 - A7 - Dm7 - D7 - G7 - Cmaj7
You will hear that the A7 you insert makes the resolution to the Dm7 'harder' and the D7 you insert makes the resolution to G7 'harder'. And of course the G7 is already a dominant chord, so it resolves very vigorously to Cmaj7.
Try it. You'll just drive all the faster down your musical highway!
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