Though two-phrase thematic units that initiate a Classical composition typically follow one of William Caplin's tight-knit models (sentence, period, hybrid), many Classical main themes demonstrate irregular construction, due to unusual patterns of cadential closure. One such theme is the "antiperiod," a two-phrase parallel unit whose second phrase cadences further off tonic that the first, whose open-ended character permits it readily to initiate a larger formal span, forming the presentation phrase of a large-scale sentence.

This paper presents three antiperiod models from Haydn and Beethoven to illustrate this theme-type in its simplest form. Subsequently, I will show how the antiperiod can comprise larger formal spans, such as Part 1 of a binary design, or a sonata exposition. The antiperiod, though relatively neglected in the literature, is a common means of thematic organization in the mid-18th century, whose influence at different structural levels persisted at least through the Classical period.