B Dominant 7th
7
0 - 4 - 7 - 10Major + minor 7th
About B Dominant 7th
B7 exists because B major is hard. Four fingers, no barre, and the same harmonic job — it leads to E — which is why generations of guitarists have learned this chord years before its plain major form. It is a workaround that became standard.
The notes
B, D♯, F♯, A. The A is the flat seventh, and it lies across the strings in a way that lets the open position work: the shape zigzags rather than barring, using the second fret on the A string, the first on the D, the second on the G and the second on the high E.
The chord contains the same tritone logic as every dominant — D♯ against A — and the same expectation. Play B7 and the ear leans toward E without being told.
The shape and why it is worth the four fingers
The fingering is awkward the first week and automatic after that. What makes it worth learning is what it replaces: a second-fret barre with five sharps behind it, at the point on the neck where barre chords are hardest.
For songs in E — which on this instrument means a great many songs — B7 is the difference between a progression a beginner can play and one they cannot.
Where it shows up
As the V7 of E major and E minor alike. In blues in E it is the turnaround chord, the one that sends the twelve bars back to the beginning.
It is also the standard ending of a blues verse: the last two bars go to B7 and stop, leaving the phrase open so the next verse can start. That unfinished quality is the chord doing exactly what its tritone promises.