Compatibility

The heart of jazz theory is chord-scale compatibility, which allows multiple players to improvise heavily scale-based solo parts over chord changes. Chords are grouped into modes with which they are said to be compatible.

This is not a hard-fast rule, just a way to remember which chords are connected to the their scalar counterparts. In other words, an experienced jazz musician can look at a series of chords and know which scales (and which notes of those scales) will sound good over the chord changes, and can figure out how to harmonize a melody by looking at the notes of the melody (horizontal axis) and determining which chord changes fit underneath (vertical axis).

(See Extensions & Avoid Notes for a detailed elaboration.)