A periodic sentence (also called a period) is a sentence that is not grammatically complete until its end. 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.
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.
The periodic sentence is effective when it is used to arouse interest and curiosity, to hold an idea in suspense before its final revelation.
This, the first stanza of Longfellow’s “Snowflakes,” is a 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 snow” finalizes the statement.
Periodic sentences are common in Greek and Latin writers such as Cicero, who is generally considered to be the Western world's master in this rhetorical device. English writers whose works are famous for their well-crafted periodic sentences include:
Only Countess Helene, considering the society of such people as the Bergs beneath her, could be cruel enough to refuse such an invitation. Berg explained so clearly why he wanted to collect at his house a small but select company, and why this would give him pleasure, and why though he grudged spending money on cards or anything harmful, he was prepared to run into some expense for the sake of good society—that Pierre could not refuse, and promised to come.