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. The chord loses none of its function and most of its difficulty, which is why it appears far more often than the plain triad.
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.