A Periodic Sentence is a sentence not grammatically complete until its end. The periodic sentence is effective when it is used to arouse interest and curiosity, to hold an idea in suspense before its final revelation. Periodicity is accomplished by the use of parallel phrases or clauses at the opening or by the use of dependent clauses preceding the independent clause; that is, the kernel of thought contained in the subject/verb group appears at the end of a succession of modifiers. It is the opposite of a nuclear sentence.

“Out of the bosom of the Air,

Out of the cloud-folds of her garment shaken,

Over the woodlands brown and bare,

Over the harvest-fields forsaken,

Silent and soft, and slow,

Descends the Moon.”

This, the first stanza of Longfellow’s “Snowflakes,” is a maximally periodic sentence. It begins with a succession of parallel adverbial phrases (“Out of the bosom”, “Out of the cloud-folds,” “Over the woodlands,” “Over the harvest-fields”), each followed by parallel modification (“of the air,” “of her garments shaken,” “brown and bare,” “forsaken,”). However, the thought is not grammatically complete until the subject/verb group “Descends the Moon” finalizes the statement.

It is this type of sentence, taught in oratory classes, from which derives the American use of the word "period" to mean the punctuation mark that the Britons call a full stop.

Grammar | Rhetoric