Functional Harmony
Understanding chord progressions begins with understanding functional harmony. Each degree of the scale yields a specific chord variety.
Chords of Cmajor

The most recognizable progression is the "Dominant" (5th chord of the key) to the "Tonic" (1st chord of the key). In the key of Cmajor, G7 resolves to Cma7. This 4th movement is the Dominant-Tonic relationship and defines how we hear chords progress. The easiest way to illustrate this is using a circle. Moving counter-clockwise displays 4ths.
Chords of Cmajor (circle in 4ths)

Additional chords are used via substitution.

Common substitutions:
  • Secondary Dominant: When dominant chords other than the "Five Chord" (G7) are used this is called "secondary dominant". These chords create tension and therefore demand stronger resolutions by moving in 4ths, for example....B7-Emi ..... E7-Ami ..... A7-Dmi ..... D7-G7 .... C7-Fma7. Secondary Dominant chords can be used in series, for example.... B7-E7-A7-D7-G7.
  • Minor Four: Fmi instead Fma7. This borrows from a minor key where the four chord is normaly minor.
  • Diminished chords: Generally used to connect chords chromatically, for example, Cma7-C#dim7-Dmi7 ..... Dmi7-D#dim7-Emi7. This creates an ascending chromatic movement. Descending diminished chords are used this way, Emi7-Ebdim7-Dmi7.
  • Tritone Substitution: A tritone sub is when chord is replaced with it's tritone (3 whole steps), for example, G7=Db7. As a result Db7 resolves to Cma7 chromatically. Tritone subs are generally dominant chords and can be used with any chord in the key.