A popular way to enrich a chord progression is by adding dominant chords to the existing ones. The dominant chords are the V as if each chord were the I in its respective key.

For example if we had the typical: C-Am-Dm-G7 the enhanced variation using secondary dominants would be C-E7-Am-A7-Dm-D7-G7.

There is more to it. Each secondary dominant can be preceded by the II chord of the assumed tonality where the V was placed.

If we take the given example C-Am-Dm-G7. The resulting progression adding the II-V of the original chords would be:

C-Bm-E7-Am-C#m-A7-Dm-Am-D7-G7.

Minor sevenths can be used in the II chords whenever chosen.

C-Bm7-E7-Am-C#m-A7-Dm-Am7-D7-G7.

The way the dominant V resolves to the tonic I is because V7 contains the tritone, the three note interval between the fourth and the seventh (e. g. F and B). As it well known the tritone is most unstable interval, the fourth tends to move towards the third while the seventh reaches for the tonic.

And besides the V7, the tritone can be found in other chords, look at the following image:

The notes in blue form the same tritone found in V7, in this case we show C as the tonic, therefore, the tritone is formed by F and B. Considering this, we can obtain different kinds of secondary dominants.

The first case shows the traditional example that conforms the natural II-V-I progression. The second case known as Dominants by Extension involves the dominant of the dominant. Sometimes noted as V/V. In the case of G7, its V7 is D7.

The third case shows a V7 that substitutes the natural V7 in the tonality. B in G7 can enharmonically be Cb in the Db7 chord which is the same sound as B, the tritone in Db/ is exactly the same as in G7, it can then substitute this chord and solve to C giving a different color due to the rest of the notes in Db7. It is called Substitute Dominant.

We have then in the image the case of Contiguous Dominants, this happens when a dominant is a second mayor distance from the natural dominant. It happens all the time in a Blues form when changing V7-IV7. And it can be also used instead of II.

The last two cases come form minor modes, the relative minor and the parallel minor. Let us explain the Relative Minor dominants. This case is a II-V-Im, but instead of solving to the relative minor (Am in the case of C) they solve to I. The example shows Bm7-E7 resolving to C, when it normally goeas to Am.

Finally we have the Interchange Modal Dominant, which once again come from II-V-Im but refering to the minor parallel. For the situation discussed here it would be Cm, the II and V7 of Cm are respectively Dm7(b5) and G7 that solve to Cm but they can also move towards C changing the mood from minor to mayor.

Once again II-V7-I is proved a very useful tool for arranging and composing, here we have another example of a composition in the style of a Jazz Standard:

It was thought as a short melodic motif to improvise melodically and harmonically acording to the cases mentioned above. The audio can be heard here: