A Dictionary of Music and Musicians/Dominant
DOMINANT is the name now given to the 5th note of the scale of any key counting upwards. Thus G is the dominant in the key of C, F in that of B♭, and F♯ in that of B. It is so called because the key of a passage cannot be distinguished for certain unless some chord in it has this note for root; for which reason also it is called in German 'Der herrschende Ton.' The dominant plays a most important part in cadences, in which it is indispensable that the key should be strongly marked; and it is therefore the point of rest in the imperfect cadence or half close, and the point of departure to the tonic in the perfect cadence or full close. [Modes.]
It also marks the division of the scale into two parts; as in fugues, in which if a subject commences with the tonic its answer commences with the dominant, and vice versa. In the sonata form it used to be almost invariable for the second subject to be in the key of the dominant, except when the movement was in a minor key, in which case it was optional for that part of the movement to be in the relative major. In lighter and simpler kinds of composition the harmonic basis of the music often alternates chiefly between tonic and dominant, and even in the most elaborate and deeply thought works the same tendency is apparent, though the ideas may be on so extended a scale as to make the alternation less obvious.[ C. H. H. P. ]