B♭ Major 6th
6
0 - 4 - 7 - 9Major + major 6th
About B♭ Major 6th
B♭6 adds a G to a B♭ major triad, and on guitar it is a barre chord with no open-string relief. Its main life is in swing rhythm playing, where it is struck and muted four times a bar, and in jazz endings, where the sixth replaces the major seventh for a drier finish.
The notes
B♭, D, F, G. The G is a major sixth above the root, and the same four notes read from G give G minor seventh — the equivalence every sixth chord carries.
That reading matters here more than usual: G minor is the relative minor of B♭, so the two chords are neighbours in the same key and substitute for one another naturally.
The sixth also sits a whole tone below the root of the chord above it, which is why B♭6 and Gm7 can trade places mid-phrase without the harmony appearing to move at all.
On the fretboard
First fret with the root on the A string, or sixth with the root on the low E. The compact four-string voicing on the middle strings is what rhythm playing uses.
Because B♭ is a horn key, the chord usually appears in ensemble settings where the guitar occupies the middle register and the bass covers the root — which suits the partial voicings well.
Where it shows up
As a tonic in B♭ major at the end of phrases, especially in swing and early jazz. It also appears in the turnaround of a blues in B♭.
In modern playing the major seventh has largely replaced it, so a recording that ends on B♭6 usually belongs to a particular era or is quoting one deliberately.