Definition:

In traditional grammar, a construction in which two or more parts of a sentence are equivalent in meaning but not parallel (or grammatically similar) in form.

Faulty parallelism most often occurs with paired constructions and items in a series.

To correct faulty parallelism, match nouns with nouns, verbs with verbs, and phrases or clauses with similarly constructed phrases or clauses.

See Examples and Observations, below.

Also see:

• Editing Exercise: Faulty Parallelism
• Parallelism
• Parallel Structure
• Sentence Completion Exercise: Parallelism
• Antithesis
• Balanced Sentence
• Headings
• List
• Outline
• Paired Construction
• Parison
• Repetition

Examples and Observations:

• Physical and mental health and wellness rest on four pillars: regular exercise, healthy diet, social interaction, and getting sufficient sleep.
  Corrected sentence:
  Physical and mental health rest on four pillars: regular exercise, healthy diet, social interaction, and sufficient sleep.
• Making Items in a Series Comparable and Parallel
  Faulty parallelism sometimes occurs because a writer tries to compare items that are not comparable:

  NOT PARALLEL
  The company offers special college training to help hourly employees move into professional careers like engineering management, software development, service technicians, and sales trainees.
  [Notice faulty comparison of occupations--engineering management and software development--to people--service technicians and sales trainees.]

  To avoid faulty parallelism, make certain that each element in a series is similar in form and structure to all others in the same series.

  PARALLEL
  The company offers special college training to help hourly employees move into professional careers like engineering management, software development, technical services, and sales.

  (Gerald J. Alred, Charles T. Brusaw, and Walter E. Oliu, The Business Writer's Companion, 6th ed. Bedford/St. Martin's, 2011)

• Parallelism With Items in a List
  Note that if you have a series of items in [a] list (whether numbered or unnumbered), the items should be parallel--the same part of speech or the same type of phrase or clause. If the first item in the list is a question, for example, all the items should be questions. If the first item is an adverbial phrase, all the items should be adverbial phrases.

  Not this

  1. We defined our purpose.
  2. Who is our audience.
  3. What should we do?
  4. Discuss findings.
  5. Our conclusions.
  6. Finally, recommendations.

  But this

  1. Define purpose.
  2. Analyze audience.
  3. Determine methodology.
  4. Discuss findings.
  5. Draw conclusions.
  6. Make recommendations.

  (Joel P. Bowman and Bernadine P. Branchaw, How to Write Proposals That Produce. The Oryx Press, 1992)
• "[I]n edited prose faulty parallelism may generally be accounted a venial sin--if the writer doesn't notice it and the reader doesn't notice it, how serious can it be?"
  (Merriam-Webster's Dictionary of English Usage. Merriam-Webster, 1994)
• Parallelism With Correlative Items
  "Correlative items in a sentence are ones indicated by pairs of conjunctions such as either . . . or, not only . . . but also, and whether . . . or.
  
  "He has either gone swimming or someone has taken him sailing is faulty parallelism . . . because the second element is not a second predicate sharing the subjectHe with the first predicate, but an independent clause with its own subject, someone. The sentence can be made grammatically correct by changing the position of either: Either he has gone swimming or someone has taken him sailing. Now the correlative elements are both independent clauses. Another solution would be He has either gone swimming or been taken sailing. Neither solution produces perfect parallelism--in the first, one verb is intransitive and the other transitive, and in the second, one verb is active and the other passive. However, both solutions are correct, and the parallelism cannot be perfected without changing the meaning. . . .
  
  "He has either gone swimming or gone sailing is precisely parallel; gone swimming and gone sailing are grammatically similar and share their relationship with he has."
  (Edward D. Johnson, The Handbook of Good English, rev. ed. Pocket Books, 1991)