Writing tip: Back to basics with commas

Theresa Bell
Learn more about the rules for using commas.

While some writers use the “if I take a breath, I need a comma” approach to making decisions about commas, there are specific rules that dictate when a comma is required.

Use a comma:

  • To join 2 independent clauses with a coordinating conjunction (e.g., and, but, or, for, nor, so). Place the comma before the conjunction. E.g., the peacocks at RRU are beautiful, and they can usually be found meandering around campus.
  • After an introductory phrase, prepositional phrase, or dependent clause. E.g., regardless of their beauty, the peacocks’ early morning vocalizations make them unpopular with students staying in residence.
  • To separate non-essential elements from a sentence. Non-defining relative clauses (also known as non-restrictive clauses) begin with a relative pronoun (e.g., who, whom, which, there). E.g., the peacocks, who have been residents at RRU since the 1960s, share the campus with other wildlife.
  • After a transition. E.g., the peacocks may be noisy; however, RRU wouldn’t be the same without them.
  • To separate elements in a series. The APA Style rules require the serial comma in a list of three or more items (American Psychological Association, 2010, p. 88), which is the comma that appears after the second-to-last item in a list. E.g., the colours in peacocks’ feathers include blue, green, copper, and black.
  • Between coordinate adjectives (i.e., adjectives that are equal and reversible). E.g., the quiet, frequent clucks of mother peahens to their chicks keep the chicks from wandering too far astray.
  • Before a quotation. E.g., The visitor asked, “Why did the peacock cross the road?”. The RRU staff member replied, “Your guess is as good as mine; peacock logic is mysterious”.
  • To set off the year in an exact date. E.g., March 25, 2015.
  • To separate groups of 3 digits in most numbers of 1,000 or more. E.g., 5,000 or 500,000

One last point: do not use a comma before a defining relative clause (also known as a restrictive clause) that begins with “that”:

  • The peacocks that walk slowly are tame and used to human company (“that walk slowly” defines or restricts which peacocks are being referred to).
  • The peacocks, which walk slowly, are tame and used to human company (“which walk slowly” provides non-essential information, and if removed, wouldn’t impact the meaning of the sentence).

For more information on choosing “that” or “which”, please see “That or which?”. For more information on commas, please see “Punctuating sentences: Commas, semicolons, and colons” in Punctuation. If you have any questions about this writing tip, please contact the Writing Centre.

Theresa Bell
Writing centre coordinator


American Psychological Association. (2010). Publication manual of the American Psychological Association (6th ed.). Washington, DC: Author.