One of the principles of standard English usage is that if a sentence presents multiple related items or phrases, then each of those items or phrases should be presented in parallel grammatical structures. Parallelism is also called parallel structure or parallel construction.

Parallelism is one of the favourites of GMAT. It involves grammar as well as logic. Suppose A and B are two phrases or clauses that you want to put in parallel. There are two things to consider - both A & B should be grammatically correct and both must match grammatical form. 

Parallel Structure

Single words, multi-word phrases and clauses can be made parallel by ensuring that each item is equal. Examples of parallel structure include multiple gerunds or –ing verbals that function as nouns, past participles, infinitive phrases (to be) and complete clauses that include a subject and predicate. Never mix and match when using parallel structure to create a list or series.

Example

  • Faulty Statement: Harold, a fervent environmentalist, is angry about the loss of wetlands, the decrease in biodiversity, and the destroying of the rainforest.

The problem here is that the first two items in the list are nouns (loss and decrease), but the third item is a gerund (destroying). The parallel form would be destruction.

Parallelism and Conjunctions

Two parallel elements (phrases or clauses) are joined by conjunctions (coordinating or correlative). The most common types are:

  • A and B
  • A or B
  • A but B
  • A as well as B
  • A rather than B
  • not only A but also B
  • both A and B
  • either A or B