B♭ Dominant 7th
7
0 - 4 - 7 - 10Major + minor 7th
About B♭ Dominant 7th
B♭7 is the chord that makes a blues in E♭ work and a blues in F turn around. Its notes — B♭, D, F, A♭ — sit entirely outside the guitar’s open strings, so the shape is a barre wherever it is played, and most guitarists learn it as a movable grip rather than a chord.
The notes
B♭, D, F, A♭. The A♭ is the flat seventh, and the tritone it forms with D is what gives the chord its pull — in the key of E♭ that pull points home.
Compared with B♭ major the difference is one note and a great deal of direction: the triad can end a phrase, the seventh cannot.
Positions
First fret with the root on the A string, or sixth fret with the root on the low E. The sixth-fret shape is easier and the one most players default to.
The ninth variant, B♭9, uses the same movable funk grip that serves every other key — one shape, twelve chords, and no open strings to complicate it.
Where it shows up
As the V7 of E♭ major and the IV7 of a blues in F. Both are horn keys, which is why this chord belongs to jazz and swing repertoire more than to guitar-written songs.
It also appears as the turnaround chord in a blues in F, where the last two bars go to B♭7 and back — the same job B7 does in the key of E.
For a guitarist the practical lesson is that this chord almost never appears alone: it belongs to keys where every other chord is also a barre, and learning it means learning the whole set at once.