Composers avoid the splitting of compositions in measures – phrases - of four and use instead sizes of four or eight measure phrases which are linked to each other. Two phrases only pare when they are melodic connected and when the second phrase ends with a strong authentic cadence. This creates a period or musical sentence.
- parallel period = when the beginning of both phrases contain the form aa or aa’.
- contrasting period = when they differ melodic, but have comparable aspects with material from the motif (ab).
- interrupted period = when only the second phrase resolves in the tonic (V//I).
- continuous period = when the dominant at the end of the first phrase moves on into the second phrase and so the second doesn’t begin with the tonic.
- sectional period = when phrase one and two end on the tonic, but only the second on the first beat of the measure or the root in soprano (and both with harmonic closure).
- progressive period = when one of the phrases modulate.
- double period = when the antecedent and consequent have in themselves an antecedent and apodosis.
When the musical phrase or sentence is short-short-long, it is aa’b. In this new sentences can be created; a=aa’b, a’= aa’b and b-cc’d. We call this nested structures.
- modified period = when this antecedent and consequent relation is broken by aab or abb.
- phrase group = when cadences are too weak to be called a period.
- asymmetrical period = uneven numbered groups of phrases like abc or aabcc.
Yours Sincerely @inMusicalTerms
with the help of The Complete Musician: An Integrated Approach to Tonal Theory, Analysis, and Listening, 3rd Edition - Steven G. Laitz