Conventional expectations for language include that words or groups of words that are working together in a coordinated way must share the same grammatical form. [EDIT]
A consistent aspect of the English language is a sense of balance. In English, we keep a balance between subjects and verbs, both plural or both singular; we keep pronouns and antecedents balanced also, both plural or both singular.
As a grammatical term, parallelism is a balance maintained between and among elements of a sentence or paragraph that are functioning in the same grammatical ways.
Consider these basic examples:
Don enjoys running, cycling, and he has a wood-working shop in his garage. (unparallel)
Don enjoys running, cycling, and wood-working. (parallel)
Two common situations that create problems with parallel structure include “not only/ but also” and “so/ that” constructions. Consider these examples:
Not only is he unfair, but he is uncaring and deceitful. (unparallel)
He is not only unfair, but also uncaring and deceitful. (parallel)
She is so angry. (unparallel, thus incomplete)
She is so angry that she is unable to explain herself. (parallel and complete)
As a stylistic technique, parallelism is a strategy employed by writers for effect. For a historical example of purposeful parallelism for effect, read Thomas Jefferson’s Declaration of Independence:
http://www.ushistory.org/declaration/document/index.htm
For an on-line resource addressing parallelism, visit:
http://owl.english.purdue.edu/owl/resource/623/01/
