The Key of A at the Piano Seat

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