In grammar, parallelism, also known as parallel structure or parallel construction, is a balance within one or more sentences of similar phrases or clauses that have the same grammatical structure.[1] The application of parallelism improves writing style and readability, and is thought to make sentences easier to process.[2]

Parallelism is often achieved using antithesis, anaphora, asyndeton, climax, epistrophe, and symphonic.[3]

:

Lacking parallelism: She likes cooking, jogging, and to read.
Lacking parallelism: "He likes to swim and running".
Parallel: She likes cooking, jogging, and reading.
Parallel: She likes to cook, jog, and read.
Parallel: "He likes to swim and run."
Parallel: "He likes swimming and running."
In the above example, the first sentence has two gerunds and one infinitive. To make it parallel, the sentence can be rewritten with three gerunds or three infinitives.

Lacking parallelism: The dog ran across the yard, jumped over the fence, and down the alley he sprinted.
Parallel: The dog ran across the yard, jumped over the fence, and sprinted down the alley.
Note that the first nonparallel example, while inelegantly worded, is grammatically correct: "cooking," "jogging," and "to read" are all grammatically valid conclusions to "She likes." The second nonparallel example is not grammatically correct: "down the alley he sprinted" is not a grammatically valid conclusion to "The dog."

In rhetoric[edit]
Further information: Parallelism (rhetoric)
Parallelism is often used as a rhetorical device. Examples:

"The inherent vice of capitalism is the unequal sharing of blessing; the inherent virtue of socialism is the equal sharing of miseries." — Winston Churchill
"Let every nation know, whether it wishes us well or ill, that we shall pay any price, bear any burden, meet any hardship, support any friend, oppose any foe to assure the survival and the success of liberty." — John F. Kennedy[3]
"...and that government of the people, by the people, for the people, shall not perish from the earth." — Abraham Lincoln, Gettysburg Address[3]
"We have petitioned and our petitions have been scorned. We have entreated and our entreaties have been disregarded. We have begged and they have mocked when our calamity came. We beg no longer. We entreat no more. We petition no more. We defy them." — William Jennings Bryan[3]

source: wikipedia