Phrase may be added to phrase, in order to increase the primary material, and to provide for greater breadth of basis, and a richer fund of resources.

The condition to be respected is, that such aggregation shall not become the ruling trait, and, by its excess, supplant the main purpose,--that of _development_.
That is, it must be held rigidly within the domain of Unity.

The student of the classic page will therefore expect to find a more or less marked family resemblance, so to speak; prevailing throughout the various phrases that may be associated upon that page.
Each additional phrase should be, and as a rule will be, sufficiently "new" in some respect or other to impart renewed energy to the movement; but--so long as it is to impress the hearer as being the same movement--there will still remain such points of contact with the foregoing phrase or phrases as to demonstrate its derivation from them, its having "grown out" of them.
This process of addition (not to be confounded with the methods of extending a single phrase, illustrated in the preceding chapter) is exhibited first, and most naturally, in the so-called Period-form.
THE PERIOD .-- The Period-form is obtained by the addition of a second phrase to the first.

It is therefore, in a sense, a double phrase; that is, it consists of two connected phrases, covering _eight ordinary measures_, or just double the number commonly assigned to the single phrase.
Each one of these phrases must, of course, have its individual cadence, or point of repose; the first--called the _Antecedent phrase_--has its cadence in the fourth measure, and the second--called the _Consequent phrase_--in the eighth measure.

The effect of the Period-form is that of a longer sentence interrupted exactly in the middle,--not unlike a bridge of two spans, resting on a central pier.