Definition

Parallelism refers to the use of grammatically equal elements in sentences and paragraphs.  We may view parallelism as a matter of balance.  When elements are balanced (parallel), the result is clear and smooth; when they are not balanced, the result is awkward and confusing.

Principal Rules

• For parallel structure, balance nouns with nouns, prepositional phrases with prepositional phrases, participles with participles, infinitives with infinitives, clauses with clauses.
  
  Not parallel:  Our report covers the details of the plan and how much it costs.
  Parallel:  Our report covers the details of the plan and its costs.
  
  Not parallel:  Our goals were to collect information, analyzing the significance of the facts, and appropriate recommendations would be made.
  Parallel:  Our goals were to collect information, to analyze the facts, and to make appropriate recommendations.  [This would also be parallel with "to collect..., analyze..., and make...." (omit the second and third uses of to).  But the repetition has its merits: see the next bulleted rule.]
  
  Not parallel:  We would appreciate your response, orally or in writing.
  Parallel:  We would appreciate an oral or a written response.
  
  
• To emphasize and clarify parallel structure, repeat a preposition, an article, the to in an infinitive, or the introductory word of a phrase or clause:
  
  > The problem lies not in the plan itself but in the method of execution.
  
  > We are pleased that you have accepted our proposal and that you will be working with us on this important project.
  
  > We are writing to clarify our position, to prevent possible misunderstandings, and to establish a foundation for cooperation.
  
  
• Use parallel structure with correlatives: either . . . or; neither . . . nor; not only . . . but also; whether . . . or; both . . . and. 
  
  Not parallel:  Whether at home or when at work, he was always busy.
  Parallel:  Whether at home or at work, he was always busy.
  
  Not parallel:  This is not only unclear but it is wordy as well.
  Parallel:  This is not only unclear but also wordy.
  [As a rule, prefer to balance not only with but also.  We could write, "This is not only unclearbut wordy" or "This is not only unclear but wordy as well"; however, the more symmetrical "not only . . . but also" construction is stylistically superior.]
  
  Not parallel:  We will either telephone you or a letter will be sent.
  Parallel:  We will either telephone you or send you a letter.

Parallelism in Lists

Parallelism is also necessary in lists. The following list, for example, is NOT parallel.

The objectives of this course are:
       > To review the rules of grammar.
       > Examining methods of developing essays.
       > How to do a research paper.

The first item in the above list is introduced by the infinitive to review, the second by the gerund examining, the third by the phrase How to.  Furthermore, the last two items do not follow from the lead-in: "The objectives of this course are . . . examining"? "The objectives of this course are . . . how to"?

A more parallel (and logical) presentation of this list would be the following:

Always take care to ensure that each item in a list follows logically from the lead-in supplied by the introductory sentence.

The Virtues of Parallel Construction

Parallel construction prevents awkwardness and promotes clarity.  Balanced or symmetrical elements also tend to be more concise.  Writers who face the challenge of stating several ideas in one sentence can often achieve this goal with remarkable clarity if they arrange these ideas in a balanced series.  This series might comprise parallel nouns or phrases, or it might comprise a more sophisticated sequence of parallel clauses.  The result is not only easier for the reader to follow, but it is also more pleasing aesthetically.