The II  V  I progression

 As the jazz progressed into the 1940s the dominant 7th chords on the II and VI position were gradually replaced with minor chords, in accordance with the natural notes in a given tonality. Moreover, almost all chords were given some sort of extra note, often a 7th. In such course of events, the old A7-D7-G7-C standard progression (in C major) became Am7-Dm7-G7-Cmaj7. The last three chords constitute the II-V-I progression in C major, which can of course be used in any other tonality using different root notes.

 Skimming through the old song sheets, we see that II-V-I could be used in many tonalities in the interim of one song. Often, the Imaj7 became a IIm7 in a new tonality, using the same root note, as shown below  Gm7-C7-Fmaj7  Fm7-Bb7-Ebmaj7.

Playing in minor tonalities and using the diatonic notes, a II-V-I progression would use IIm7b5 instead of IIm7. Furthermore, a diatonic 7th on top of the Im chord would give a minor chord with a major 7th that is a very tense chord. Usually such chord was omitted, and substituted with minor 7th or minor 6th. Lastly, the V7 chord was often coloured with minor 9th, thus creating a leading note to the 5th of the Im7 chord. Here, we see a IIm7b5-V7b9-Im7 progression in C minor.

 It was not unusual to end a II-V-I progression on a chord of parallell tonality, e.g. IIm7-G7-Im7.
 Additionally, such ending chord could automatically be used as a IIm7 chord in a new tonality. It was also common to skip the I chords, and play a cycle of II-V progressions through different tonalities, often following the circle of fifths, e.g. Dm7-G7-Cm7-F7-Bbm7-Eb7.

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