The dominant of F♯, sitting a fourth below the chord it resolves into.
Index finger barres the A and G strings at the 4th fret
Load C♯7 onto an interactive fretboard, hear every note, and build it into a progression.
Play C♯7 in the Studio →A C♯7 chord contains the notes C#, G#, B, F. It is built from the dominant 7th formula 1 - 3 - 5 - ♭7, with C# as the root.
The flattened seventh forms a tritone with the chord's third, an interval unstable enough that the ear hears it as a question. Resolving up a fourth lets both notes settle by a semitone at once, which is why the V7 to I move sounds so final. Take the seventh away and the chord is content to sit still.