The Key of Ab at the Guitar Seat

Called in Ab, a tune has Ab – Bbm – Cm – Db – Eb – Fm – G° available to it before anybody substitutes anything; the parallel minor, G# minor, offers G#m – A#° – B – C#m – D#m – E – F# instead. The two-five-one that closes most choruses is built out of three of those seven.

The guitar seat carries the comp — the changes stated in three and four notes behind whoever is playing the melody, which in practice means dropping the root and playing the notes nobody else is covering — the third and the seventh first, everything else only if it fits under the hand.

Learn where the ii, the V and the I fall in Ab until the hand goes there without asking, and the substitution rules become edits to something you already know rather than new material.