B Minor 7th
m7
0 - 3 - 7 - 10Minor + minor 7th
About B Minor 7th
B minor seventh is what B minor becomes when a guitarist gives up on the full barre. Lift the fingers holding the fifth and the chord loses none of its function and most of its difficulty — which is why it appears in songs far more often than the plain triad it comes from.
The notes
B, D, F♯, A. The A is the seventh, and its presence is what allows the simplified shapes: the chord tolerates missing notes better than a triad does, because four voices leave room to drop one.
The chord shares three notes with D major, which is why Bm7 and D substitute for each other in arrangements, and why a song in D can sit on either without changing colour much.
Voicings
The two-fret partial shape on the top four strings is the common form. The full barre at the second fret is the complete version, and the seventh-fret A-string voicing is the movable one.
For songs in D or A, the partial shape is usually enough: the bass supplies the root, and the guitar covers the colour.
Where it shows up
As the vi of D major, the ii of A, and the iii of G. All three are guitar keys, so the chord appears constantly — usually as a passing colour rather than a destination.
In bossa nova and soul it is often the opening chord of a phrase, where its softness sets up something firmer after it.
It is also the chord that makes a full B minor optional in most songs. If the progression moves quickly, the partial seventh shape covers the same ground and leaves the hand ready for whatever follows — which is why many guitarists play it for years before mastering the barre.