Scales for improvising · Music theory
Which scale is usually played over each chord: the modes of II – V – I, dominants toward minor, the tritone substitute and altered dominants.
One scale per chord
Improvising over a progression means knowing, at every moment, which notes fit. Modern jazz organized that question into one idea: each chord has a scale that contains it and also carries its tensions. This is the chord-scale theory, systematized in the second half of the 20th century.
Within a major key, each degree takes the mode that starts on it: Dm7, Dorian; Em7, Phrygian; FMaj7, Lydian; G7, Mixolydian; Am7, Aeolian; Bm7(♭5), Locrian. They are always the notes of the same scale, but the center of gravity shifts. I takes Ionian and also Lydian, which avoids F, the hardest note over it.
A shortcut for finding each mode
To know the notes of a mode on any pitch, find the major scale that contains it. The distance is always the same, so there are no new key signatures to memorize: G Dorian has the notes of F major, and A Mixolydian, those of D major.