B♭ Major 7th
maj7
0 - 4 - 7 - 11Major + major 7th
About B♭ Major 7th
B♭ major seventh is the first chord of a great many jazz standards, and on guitar it is a barre with no open-string escape. Its notes — B♭, D, F, A — belong to the horn keys, and the chord is learned as a movable shape rather than as a position.
The notes
B♭, D, F, A. The A is the major seventh, a semitone below the octave, and it gives the chord the unhurried quality every maj7 has.
Drop the A to A♭ and the chord becomes B♭7, which pulls toward E♭. The distance between a resting chord and a moving one is again one fret.
On the fretboard
Sixth fret with the root on the low E string, or first fret with the root on the A. The sixth-fret version is the practical one; the first fret is used when the low B♭ is needed.
The four-string voicing on the middle strings is what jazz comping uses, because it leaves the bass register free and moves easily into the next chord of the progression.
The barre is also less punishing here than at the first fret, and that difference is worth naming: the same shape two frets lower is the notorious F, and players who avoid one often manage the other without noticing the inconsistency.
Where it shows up
As the I of B♭ major — the most common key in jazz repertoire — and as the IV of F. A large share of standards open on this chord.
For a guitarist it is worth learning early despite the barre, because the keys around it are unavoidable in any playing situation involving horns.