Period Form

Whenever composers and theory teachers talk about crafting a balanced melody, they always go to period form. Yes, it is talked about excessively, and many composers, especially in the Romantic era ignore it for more asymmetrical or looser structures, but it is one of the best ways to start when one is learning initially, and it was used by the masters, e.g. Bach, Mozart, and Beethoven.

The structure of a period can be pared down to this: question and answer, or the antedecent and the consequent.

 The antecedent, ideally four bars. introduces the main melodic motif in bars 1-2, and introduces a contrasting idea in the next two, leading to an inconclusive cadence, usually half or imperfect authentic.

 The consequent, the same length as the antecedent restates the opening idea, either identically or with a slight variation to suit the preceding or succeding ideas and harmonies. The contrasting idea follows the same contour as its mirror in the antedecent, however it leads to a strong, usually perfect authentic cadence. 

Depending on how it is constructed, the period has two main types. The parallel and the contrasting. In the parallel, the consequent begins in the same vein as the antedecent. Think of the opening of Mozart's K. 331 sonata. In a contrasting period, the consequent begins with new melodic material, but retains the harmonic hierarchy of an inconclusive cadence followed by a conclusive one.

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