The chord-scale system is a method of matching, from a list of possible chords, a list of possible scales widely used since the 1970s and "generally accepted in the jazz world today". The system is an example of the difference between the treatment of dissonance in jazz and classical harmony: "Classical treats all notes that don't belong to the chord...as potential dissonances to be resolved...Non-classical harmony just tells you which note in the scale to [potentially] avoid..., meaning that all the others are okay". The chord-scale system may be compared with another common method of improvisation, where one scale on one root note is used throughout all chords in a progression (for example the blues scale on A for all chords of the blues progression: A7 E7 D7).

In contrast, a different scale is used for each chord in the progression (for example Mixolydian scales on A, E, and D for chords A7, E7, and D7, respectively). Improvisation approaches may be mixed, such as using "the blues approach" for a section of a progression and using the chord-scale system for the rest. The sixteen scales commonly used today consist of the seven modes of the major scale, the seven modes of the melodic minor scale, the diminished scales, the whole-tone scale, and pentatonic and bebop scales.

In the example below featuring C7#11 and C lydian dominant every note of the scale may be considered a chord tone while in the example above featuring A7 and A mixolydian the scale is thought of as a 'filling in' of the steps that are missing between members of the chord. Students now typically learn as many as twenty-one scales, which may be compared with the four scales commonly used in jazz in the 1940's (major, minor, mixolydian, and blues) and the two later added by bebop (diminished and whole-tone) to the tonal resources of jazz. Originating with George Russell's Lydian Chromatic Concept of Tonal Organization (1959), the chord-scale system is now the "most widely used method for teaching jazz improvisation in college".

This approach is found in instructional books including Jerry Bergonzi's Inside Improvisation series and characterized by the highly influential Play-A-Long series by Jamey Aebersold. There are differences of approach within the system. For example, Russell associated the C major chord with the lydian scale, while teachers including John Mehegan, David Baker, and Mark Levine teach the major scale as the best match for a C major chord.

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