Comma usage is in some respects a question of personal writing style: some writers use commas liberally, while others prefer to use them sparingly. Most modern North American style guides now recommend using fewer commas rather than more, so when faced with the option of using a comma or not, you may find it wise to refrain.
For instance, the use of a comma before the and in a series is usually optional, and many writers choose to eliminate it, provided there is no danger of misreading the sentence, as in the following examples:
- We bought scarves, mittens and sweaters before leaving for Iceland. (comma unnecessary before and)
- We ate apples, plums, and strawberry and kiwi compote. (comma needed before and for clarity)
Use a comma before a co-ordinating conjunction that joins independent clauses (unless the independent clauses are very short):
- I wrapped the fresh trout in three layers of newspaper, but my van still smelled like fish for the next week. (comma with two independent clauses)
- She invited him to her party and he accepted. (comma unnecessary with short clauses)
- 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. (comma after introductory adverb clause)
- From the east wall to the west, the cottage measures twenty feet. (comma after introductory prepositional phrase)
- In the bottom drawer you will find some pink spandex tights. (no comma with short, closely related phrase)
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. (the comma preceding and is optional unless needed to prevent misreading)
Use commas to set off non-restrictive elements and other parenthetical elements. A non-restrictive 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.
- 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.
- Mr. Grundy’s driving record (with one small exception) was exemplary.
- Mr. Grundy’s driving record—with one exception—was exemplary.
Equally important in understanding how to use commas effectively is knowing when not to use them. While this decision is sometimes a matter of personal taste, there are certain instances when you should definitely avoid a comma.
Do not use a comma to separate the subject from its predicate:
- [WRONG] Registering for our fitness programs before September 15, will save you thirty percent of the membership cost.
- [RIGHT] Registering for our fitness programs before September 15 will save you thirty percent of the membership cost.
Do not use a comma to separate a verb from its object or its subject complement, or a preposition from its object:
- [WRONG] I hope to mail to you before Christmas, a current snapshot of my dog Benji.
- [RIGHT] I hope to mail to you before Christmas a current snapshot of my dog Benji.
- [WRONG] She travelled around the world with, a small backpack, a bedroll, a pup tent and a camera.
- [RIGHT] She travelled around the world with a small backpack, a bedroll, a pup tent and a camera.
Do not misuse a comma after a co-ordinating conjunction:
- [WRONG] Sleet fell heavily on the tin roof but, the family was used to the noise and paid it no attention.
- [RIGHT] Sleet fell heavily on the tin roof, but the family was used to the noise and paid it no attention.
Do not use commas to set off words and short phrases (especially introductory ones) that are not parenthetical or that are very slightly so:
- [WRONG] After dinner, we will play badminton.
- [RIGHT] After dinner we will play badminton.
Do not use commas to set off restrictive elements:
- [WRONG] The fingers, on his left hand, are bigger than those on his right.
- [RIGHT] The fingers on his left hand are bigger than those on his right.
Do not use a comma before the first item or after the last item of a series:
- [WRONG] The treasure chest contained, three wigs, some costume jewellery and five thousand dollars in Monopoly money.
- [RIGHT] The treasure chest contained three wigs, some costume jewellery and five thousand dollars in Monopoly money.
- [WRONG] You should practice your punches, kicks and foot sweeps, if you want to improve in the martial arts.
- [RIGHT] You should practice your punches, kicks and foot sweeps if you want to improve in the martial arts.
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