Rule: WHICH is used with a comma to introduce a non-essential clause (can be left out without changing the meaning of a sentence).

  • Correct:  Brush your teeth with toothpaste containing abrasives, which constitute about 50 percent of typical toothpaste.
  • Incorrect: Brush your teeth with toothpaste containing abrasives that constitute about 50 percent of typical toothpaste.


Rule: THAT is used without a comma to introduce an essential clause (one that is essential to the meaning of a sentence).

  • Correct:  Dogs that are hypoallergenic tend to shed and affect allergy sufferers less.
  • Incorrect: Dogs, which are hypoallergenic, tend to shed and affect allergy sufferers less.

Rule: THAT is used as a conjunction to introduce a dependent clause when a sentence sounds or looks awkward without it.
  • Example: The woman said Tuesday that she had left her purse at home.

Rule: After some verbs, THAT is usually required:
  • assert
  • contend
  • declare
  • estimate
  • make clear
  • point out
  • propose
  • state