If we start with F as key-note, besides a small difference at we have as the fourth from it 3 X 4 = y, making with B = I R 5 an interval and requiring a new note, B flat.
If we take the new note B flat as key-note, another note, E flat, is required.
He chose a sprightly sonata in b-flat major.
Finally, Mike and Duncan performed a blues { in B flat }, with drum accompaniment.
The 2nd subject of a sonata form movement is usually in the dominant, in this case B flat major.