The dominant key in a given musical composition is the key whose tonic is a perfect fifth above (or a perfect fourth below) the tonic of the main key of the piece. Put another way, the key whose tonic is the dominant scale degree in the main key.
If, for example, a piece is written in the key of C major, then the key of C is the tonic key. The key of G major is the dominant key since it is based on the dominant note for the key of C major.
In sonata form in major keys, the second subject group is usually in the dominant key. Even with the widest roaming modulations in the development, the dominant key exerts influence and eventually forces a return to the tonic key.
Famous quotes containing the words dominant and/or key:
“Combativeness was, I suppose, the dominant trait in my grandmother’s nature. An aggressive churchgoer, she was quite without Christian feeling; the mercy of the Lord Jesus had never entered her heart. Her piety was an act of war against Protestant ascendancy. ...The teachings of the Church did not interest her, except as they were a rebuke to others ...”
—Mary McCarthy (1912–1989)
“Woe unto you, lawyers! for ye have taken away the key of knowledge: ye entered not in yourselves, and them that were entering in ye hindered.”
—Bible: New Testament Jesus, in Luke 11:52.