The full-width dominant that resolves into B.
Index finger barres the top, D and low E strings at the 2nd fret
Load F♯7 onto an interactive fretboard, hear every note, and build it into a progression.
Play F♯7 in the Studio →A F♯7 chord contains the notes F#, C#, E, A#. It is built from the dominant 7th formula 1 - 3 - 5 - ♭7, with F# as the root.
When you want the chord to carry the low end. The six-string form puts the root on the bottom string, which suits a guitar playing alone or leading a change. If a bass is already covering that register, the lighter five-string voicings leave more room and sound less congested.