The Key of Bb at the Guitar Seat

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