Another type of phrase structure, the period, arose in the Baroque era of Western tonal music (around 1600–1750). It has remained in use ever since.

Whereas a sentence is a single phrase, a period comprises two phrases. The first phrase is called the antecedent; the second phrase is called the consequent.

Because there are two phrases, there are also two cadences. In order to form a proper period, the first cadence must be of weaker strength than the second. The cadences listed in order of diminishing cadence strength are: PAC, IAC, and HC. Therefore, there are three possibilities for cadence pairs in a period: HC/IAC, HC/PAC, IAC/PAC.

There are two main types of periods: parallel periods and contrasting periods. Parallel periods feature an antecedent and consequent that begin the same or similarly. Contrasting periods feature an antecedent and consequent that begin differently.

The image below shows a phrase diagram for a parallel period, which includes the following components:

A large arc for the entire period.
Above the large arc, a label for the parallel period.
Two phrase arcs, one for each phrase.
Above each phrase arc, two labels: a and a'. The first "a" stands for the antecedent; the " a' " label stands for the consequent. The prime symbol is used because the end of the consequent must be different from the end of the antecedent. This is because the cadences must be different.
Labels for each cadence at the end of both phrase arcs. The example below shows "HC" and “PAC," though recall that HC/IAC and IAC/PAC cadence pairs are possible.
Measure numbers below the phrases that indicate how long each phrase is. If measure numbers are not provided for the music you are diagramming, you can use numbers beginning on 1. If measure numbers are provided, use the actual measure numbers.

A phrase diagram for a contrasting period is largely the same, but with one important difference. The antecedent and the consequent must be labeled "a" and "b," respectively, because the phrases do not resemble each other.

One intriguing possibility: because a sentence is a single phrase and a period contains two phrases, it is possible for one or both of the phrases in a period to be a sentence. In this case, the resulting diagram must include all of the elements for both a period diagram and a sentence diagram.