Parallelism may be defined as the similarity of structure between related words, phrases or clauses. Parallelism is frequently used to persuade someone of something, because the repetition structure is convincing by nature. The form may also be used to improve the flow of a work, allowing for a smoother discussion of ideas. The technique is not limited to writing, frequently being used for oral conversations as well. Parallelism may also be called parallel structure or isocolon.
Antithesis is a type of parallelism in which an author has grouped opposite ideas into a similar structure. For example, Alexander Pope's line "To err is human; to forgive divine" employs antithetic parallelism by establishing a human and divine trait using similar sentence structure.
The repetition inherent in parallelism may also be used for emphasis, such as in the famous opening to Charles Dickens's "A Tale of Two Cities." "It was the best of times, it was the worst of times" naturally compels the reader to wonder what "it" is, focusing on the era of the literary work. The technique may also be used to make poetry more rhythmic or for dramatic effect on a stage.
The most common example of parallelism is a list, which generally groups nouns with other nouns, -ing words with other -ing words, or some other like group. Failure to present lists in this fashion is called faulty parallelism and considered a violation of the rules of traditional grammar.