Most of the descriptions and examples in this section are taken from William Strunk's venerable Elements of Style, which is maintained online by the Bartleby Project at Columbia University:

This principle, that of parallel construction, requires that expressions of similar content and function should be outwardly similar. The likeness of form enables the reader to recognize more readily the likeness of content and function. Familiar instances from the Bible are the Ten Commandments, the Beatitudes, and the petitions of the Lord's Prayer.

Students should also visit the section on Sentence Variety, which has material on the repetition of phrases and structures. Click HERE to visit a page containing the biblical passages mentioned above. Also in this Guide is a definition of the idea of a college, a lovely example of parallel form. Students are also familiar with Abraham Lincoln's Gettysburg Address, which abounds with examples of parallel form. Clicking on the title above will allow you to read this famous speech and view a brief "slide-show" demonstration of the parallel structures within Lincoln's famous text. (The Library of Congress maintains a site at which you can inspect two different drafts of the Gettysburg Address in Lincoln's own handwriting.)

Unskillful writers often violate this principle, from a mistaken belief that they should constantly vary the form of their expressions. It is true that in repeating a statement in order to emphasize it writers may have need to vary its form. But apart from this, writers should follow carefully the principle of parallel construction.

Faulty ParallelismCorrected Version
Formerly, science was taught by the textbook method, while now the laboratory method is employed. Formerly, science was taught by the textbook method; now it is taught by the laboratory method.

The left-hand version gives the impression that the writer is undecided or timid; he seems unable or afraid to choose one form of expression and hold to it. The right-hand version shows that the writer has at least made his choice and abided by it.

By this principle, an article or a preposition applying to all the members of a series must either be used only before the first term or else be repeated before each term.

Faulty ParallelismCorrected Version
The French, the Italians, Spanish, and Portuguese The French, the Italians, the Spanish, and the Portuguese
In spring, summer, or in winter In spring, summer, or winter (In spring, in summer, or in winter)

Correlative expressions (both, and; not, but; not only, but also; either, or; first, second, third; and the like) should be followed by the same grammatical construction. Many violations of this rule can be corrected by rearranging the sentence.

Faulty ParallelismCorrected Version
It was both a long ceremony and very tedious. The ceremony was both long and tedious.
A time not for words, but action A time not for words, but for action
Either you must grant his request or incur his ill will. You must either grant his request or incur his ill will.
My objections are, first, the injustice of the measure; second, that it is unconstitutional. My objections are, first, that the measure is unjust; second, that it is unconstitutional.

When making comparisons, the things you compare should be couched in parallel structures whenever that is possible and appropriate.

Faulty ParallelismCorrected Version
My income is smaller than my wife.My income is smaller than my wife's.

The inherent vice of capitalism is the unequal sharing of blessings; the inherent virtue of socialism is the equal sharing of miseries.

Winston Churchill

QuizQuiz on Parallel Structures

QuizSecond Quiz on Parallel Structures