The Key of C at the Guitar Seat

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