From Open Music Theory (Shaffer, 2019):

“The period is generally eight measures long and contains two four-measure phrases, called antecedent and consequent…

The period is characterized by balance and symmetry. Its antecedent phrase is initiated by a basic idea that recurs at the beginning of the consequent phrase. Unlike the sentence, which exhibits a single cadence, the period contains two cadences, a weak one to end the antecedent and a strong one to end the consequent.”

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The period is a tight-knit thematic type that contains two phrases (thus, two cadences). The cadences are in a weak-strong relationship to each other.

Some call this the “parallel” period because the two phrases begin with similar material.

Because the period contains two phrases, each has a beginning+middle+end.

More broadly, each phrase also serves a function:

• The antecedent begins the theme by presenting a basic idea – “a call.” As it moves toward a cadence, however, you hear that the theme is incomplete (as the cadence is usually a half-cadence or IAC)
• The consequent follows suit, repeating the basic idea as a “response” to the lack of closure. This leads toward a second, stronger, cadence that satisfactorily resolves the harmonic tension of the theme.

Mozart – Sonata in B♭ Major, K. 281, Mvmt. I
(Recording on youtube)