B7 - Si Dominant Seventh

In music theory, B dominant seventh chord is built on B-D#-F#-A formation: root-major third-minor third-minor third. This basic dominant seventh chord has a formation B-D#-A-B-F# from basses to trebles in the guitar.

This chord is derived from a seventh chord. In music theory, dominant seventh chords are built by four different notes. But in the guitar, generally basic seventh chords are expanded by same kind notes that repeat themselves in different octaves. Dominant explains chords function on a different tone and indicates this chord is a fifth degree chord on a tone(C major tone: C Major - Tonic Degree : G dominant seventh - Dominant degree). And generally it has a unique function as resolving harmony to the tonic degree(V7 - I).