A musical period consists of two parts, each consisting of two phrases of equal length with one, two or four bars. The first part is regarded as the opening and is referred to as the antecedent, the second part is considered the closure and is called the consequent. The first phrase in both sentences is mostly identical. The antecedent often ends with a half cadence, e.g. with the dominant, the second with an authentic cadence, e.g. the root note or its octave.

Model of a period according to Ulrich Kaiser

A famous example is Mozart, Wolfgang Amadeus's Piano Sonata K. 331: