The Key of G at the Piano 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 piano seat carries the whole harmony at once — a shell in the left hand, the colour tones and any substitution in the right, which is why this is the seat a substitution is usually heard from first. It is the only seat here that can state a chord and its substitute in the same beat, left hand holding one and right hand implying the other.

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.