About the B7 chord
To play B dominant 7th, use the first shape above. Use your index finger to barre the 2nd fret across the G, C, E, and A strings. Place your middle finger on the 3rd fret of the C string. Strum all four strings and check that each one rings cleanly.
Root
B
Quality
Dominant 7th
Notes
B · D# · F# · A
How the B7 chord works
B
Root(1)
The home note. It gives the chord its name.
D#
Major third(3)
The major-third interval that establishes the chord's major base.
F#
Perfect fifth(5)
Adds strength and stability.
A
Minor seventh(♭7)
Adds the defining minor-seventh color.