When and Where to Use Commas

Before co-ordinating conjunctions joining independent clauses

Use a comma before a co-ordinating conjunction that joins independent clauses (unless the independent clauses are very short):

I wrapped the fresh fish in three layers of newspaper, but my van still smelled like trout for the next week. (commas with two independent clauses)

She invited him to her party and he accepted. (comma unnecessary with short clauses)

After introductory adverb clauses and introductory phrases

Use a comma after an introductory adverb clause and, often, after an introductory phrase (unless the phrase is very short):

After the hospital had completed its fund-raising campaign, an anonymous donor contributed an additional $10,000. (after introductory adverb clause)

From the east wall to the west, her cottage measures twenty feet. (after introductory prepositional phrase)

In the bottom drawer you will find some pink spandex tights. (no comma with short, closely related phrase)

Items in a series

Use a comma to separate items in a series:

Playing in a band can be exciting, but many people do not realize the hardships involved: constant rehearsals, playing until 2 a.m., handling drunken audience members, and transporting heavy equipment to and from gigs.

Note: the comma preceding "and" is usually optional unless needed to prevent misreading. However, there are some formatting styles (such as the "Oxford" formatting style) and instructors that require you to use the comma before the conjunction "and." Some people may refer to this as the "Oxford comma" or the "serial comma." Ask your instructor for clarification or, to be safe, always place a comma before the word and in a series.

Setting off parenthetical elements

Use commas to set off nonrestrictive elements and other parenthetical elements. A nonrestrictive modifier is a phrase or clause that does not restrict or limit the meaning of the word it is modifying. It is, in a sense, interrupting material that adds extra information to a sentence. Even though removing the non-restrictive element would result in some loss of meaning, the sentence would still make sense without it. You should usually set off non-restrictive elements with commas:

The people of Haiti, who for decades have lived with grinding poverty and mind-numbing violence, are unfamiliar with the workings of a true democracy.

I want to thank my uncle, Jim Smith, for all his support.

The request, which I submitted before the deadline, was rejected.

I have found the website, which I have been searching for.

A restrictive modifier is a phrase or clause that limits the meaning of what it modifies and is essential to the basic idea expressed in the sentence. You should not set off restrictive elements with commas:

The request that I submitted before the deadline was rejected. | The request I submitted before the deadline was rejected.

I have found the website that I have been searching for. | I have found the website I have been searching for.

Those residents of Ottawa who do not hold secure, well-paying jobs must resent the common portrayal of the city as a land of opportunity.

Note that you can use two other punctuation marks to set off non-restrictive elements or other parenthetical information: parentheses and dashes. Enclosing parenthetical information in parentheses reduces the importance of that information:

Mr. Grundy's driving record (with one small exception) was exemplary.

Placing parenthetical information between dashes has the opposite effect: it emphasises the material:

Mr. Grundy's driving record—with one exception—was exemplary.

Nevertheless, you should usually set off parenthetical information with commas.

Dates

Use commas in certain date formats.

A comma is used to separate the names of days of the week, from the month, and the date from the year:

April 17, 1982, was an important day in Canadian history.

I remember well Saturday, June 13, 1988.

On Friday, June 1st at six o’clock, we'll have a small meal for everyone that helped us move.

Please visit us on Saturday, April 1st, 2047, to participate in the reveal of the time capsule.

You do not need to use a comma when using a day-month-year format or if you are only providing month and a year:

The celebrations will begin on 1 July 2030.

The store hasn't seen this much traffic since January 2020.

In May 2020, Québec experienced a massive heatwave.