Bb Dominant 7

Bb Dominant 7, written Bb7, is built from Bb, D, F and Ab: the root, major 3rd, 5th and minor 7th.

Its 3rd and 7th, D and Ab, are its guide tones: they say what kind of chord it is, so voicings are built around them and the root is often left to the bass. E7, a tritone away, has the same two notes the other way up, so it can stand in for Bb7 as its tritone substitute.

In the progressions here, Bb7 is the V7 of the Major ii-V Progression in Eb, the ♭VII7 of the Backdoor ii-V-I Progression in C, the ♭II7 of the Tritone Sub ii-V-I Progression in A, the VI7 of the Major I-VI-ii-V Progression in Db, the ♭III7 of the Tritone Sub Turnaround in G and the III7 of the Cycle of Dominants in Gb.