[Illustration: Example 44. Fragment of Mendelssohn.]
The co-operation, or interaction, of the principles of Unity and Variety, is nowhere more strikingly shown than in the formulation of the musical period. Either element has the right to predominate, to a reasonable degree, though never to the exclusion or injury of the other. In the above example, the principle of Unity predominates to a somewhat unusual extent:—not only the figures (marked 1-2-3-4), and the motives (a-b), are uniform, in the Antecedent phrase itself, but the melody of the Consequent phrase corresponds very closely throughout to that of the Antecedent, only excepting a trifling change in the course (marked N. B.), and the last few tones, which are necessarily so altered as to transform the semicadence into a perfect cadence. It is this significant change, at the cadence, which prevents the second phrase from being merely a "repetition" of the first one,—which makes it a "Consequent," a response to the one that precedes.
Further (Mendelssohn, No. 23):—
[Illustration: Example 45. Fragment of Mendelssohn.]
In this example also, the Consequent phrase is a complete affirmation of its Antecedent, agreeing in its melodic form with the latter until the cadence is nearly due, when an extra measure is inserted (as extension), and the usual digression into the necessary perfect cadence is made. The condition of Unity predominates, but a noticeable infusion of Variety takes place.
Further (Mozart, pianoforte sonata):—