Chords in Bb major

Bb major is built from the notes Bb, C, D, Eb, F, G and A. Its scale carries 2 flats. Harmonising that scale in thirds gives seven chords — 3 majors, 3 minors and 1 diminished — starting on Bb as the I. The chord on the fifth degree is F, a major V, so Bb major has the leading tone that makes a F-to-Bb cadence sound final.

Beyond those seven, Bb major commonly borrows Bbm (i), Cdim (iiø), Db (bIII), Ebm (iv) and Fm (v) from the parallel minor — the mixture chords that give a progression somewhere unexpected to go.