Melodic Phrases and Harmonic Cadences

Motives, Phrases, and Periods

A composer creates a piece by deciding what the building blocks will be, how they will be developed and extended, and what form the piece will take.  The smallest of these building blocks is the motive.

Motives are typically combined and varied to create a phrase.  The phrase might be referred to as the smallest structural unit that is concluded with a cadence.  Two or more phrases together can be used to build a period, if the cadence in the middle is weaker or less conclusive than the final cadence.

Types of Cadences

A harmonic cadence is a sequence of chords that end a phrase.  The function of the last two chords define the cadence.  Common types of cadences are listed below.

PAC – Perfect Authentic Cadence: V-I  progression,  both in Root Position, with tonic in Soprano
IAC – Imperfect Authentic Cadence: V-I  progression that does not meet all criteria for PAC, or vii-I 
HC – Half Cadence: any cadence that ends on V, or the Dominant chord
DC – Deceptive Cadence: V followed by some chord other than I, usually vi or VI.
PC – Plagal Cadence: IV-I progression

The example below shows a symmetrical period with a Half Cadence ending the first phrase, and a Perfect Authentic Cadence ending the second phrase.  A formal diagram shows the architecture: 

Musical example of a Contrasting Period

Diagram of a Contrasting Period

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