The dominant is the “tension” that demands release into the tonic. The pair dominant → tonic (V–I) is the most powerful progression in Western music, called the authentic cadence.
The dominant seventh chord (V7) adds even more pull: the seventh wants to descend by a semitone, the third to rise by a semitone. Together they create the magnetic effect of coming home. Jazz expanded this function, creating a whole vocabulary of altered dominants (V7♭9, V7♯11, V7alt — each with its own color).
The dominant is the fifth degree of the scale, and its power is rooted in acoustics: in the natural overtone series the dominant appears among the first pitches, so the ear hears the motion V→I as the most natural “coming home.” The leading tone (the 7th degree) built above the dominant reinforces this pull, straining upward by a semitone to the tonic. Without the tension of the dominant, classical harmony would lose its sense of motion and goal.
