The Key of G at the Guitar Seat

Called in G, a tune has G – Am – Bm – C – D – Em – F#° available to it before anybody substitutes anything; the parallel minor, G minor, offers Gm – A° – Bb – Cm – Dm – Eb – 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 G until the hand goes there without asking, and the substitution rules become edits to something you already know rather than new material.