B Major 7th
maj7
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About B Major 7th
B major seventh shows how much easier a seventh can be than its triad: four strings, index finger free, no full barre. B major demands a barre at the second fret with five sharps behind it; this version sounds richer and costs less.
The notes
B, D♯, F♯, A♯. The A♯ sits a semitone below the octave B, and it is the note that turns a demanding barre chord into something that hovers rather than declares.
Lower it to A and you get B7, the chord guitarists reach for instead of plain B. All three — B, B7, Bmaj7 — live within two frets of each other and do entirely different jobs.
On the fretboard
The four-string voicing starting at the fourth fret of the D string avoids the barre altogether. The full barre at the second fret is the complete version and is rarely necessary.
At the seventh fret with the root on the low E, the standard movable major seventh shape applies. That is where the chord is usually played when it needs to be part of a line rather than a strum.
Where it shows up
As the I of B major and the IV of F♯. Both are keys the guitar meets through transposition rather than by choice, which is why the chord appears mostly in arrangements written elsewhere.
In jazz it closes the ii–V–I in B, and in bossa nova it appears often, because that idiom favours keys the guitar finds awkward and voicings that avoid the bass. It appears most often as a resting chord at the end of a phrase, where the seventh keeps the harmony from closing too firmly.