Using correct punctuation is important if you want your readers to understand what you are talking about. Of all the punctuation marks in English, the comma is perhaps the most troublesome. Omitting a comma when one is needed might completely change the meaning of a sentence, while using too many commas can be jarring and throw off the rhythm of your writing.
Many writers find themselves unsure of when to use this mark, but there are only a few rules to abide by.
Are you writing a list?
Generally, people know that between the items in a list there must be a comma. For example:
The ribbons were green, orange, and fuchsia.
The problem arises with that final comma before “fuchsia.” Known as the Oxford comma or the serial comma, it is more common in the United States than in the United Kingdom. This is what the sentence looks like without it:
The ribbons were green, orange and fuchsia.
If you were to ask Strunk and White, they would say the serial comma is always correct except in company names without it. In addition, MLA and APA prefer the serial comma, but AP Style does not.
It is up to you to decide if you like the serial comma. If you are writing to a prescribed style, you don’t have a choice, but if you do, consider this:
There is a time when you must use the serial comma. This is when one of the items in the list has an “and” in it—so, there is more than one “and” in the list. For example,
Beth bought salami, ham and cheese, and peanut butter and jelly sandwiches.
If you remove the final comma—the serial comma—, it is unclear what exactly the sandwiches have on them. Is it a ham and cheese and peanut butter sandwich? Or did someone put cheese on the PB&J? We don’t know for sure what kinds of sandwiches Beth bought unless the comma is included.
Are you joining together two full sentences?
If you are, you must be sure to also use a conjunction with the comma. Think of the comma as half a period, and a conjunction as the other half of the period. They work together to fully separate the two thoughts. If you only use the comma, you have a comma splice. Be careful here. The comma and the conjunction are not always together. Here are some examples of comma-conjunction coordination:
Judy went to work yesterday, but John went back to sleep.
When Judy went to work yesterday, John went back to sleep.
Judy went to work yesterday, while John went back to sleep.
Judy went to work yesterday, so John went back to sleep.
After Judy went to work yesterday, John went back to sleep.
Because John went back to sleep, Judy went to work yesterday.
Judy went to work yesterday because John went back to sleep.
This last one is a bit tricky. When the “because” statement goes after the “effect” statement, there is no comma.
Are you including “extra” information in the sentence?
Read your sentence out loud several times. Try to strip it down. Could you take out some of the words? Do you, perhaps, have a clause or introductory word that is nonessential to the sentence? It needs commas around it, then. This rule applies to a variety of comma usages, including introductory phrases (After lunch, Sam went…), interjections (Oh, this pasta is delicious), nonrestrictive clauses (my mother, who likes to knit, …), and nonrestrictive appositives (my friend, John, …).
The comma hugs the nonessential extra information, keeping it separate from the rest of the sentence while not completing shutting it out. Some people put parentheses around such statements (like this), but this is generally not correct. Parentheses are often skipped over by a reader’s eye, and if you have something to say, say it—don’t hide it. If commas hug words, then parentheses wrap them up completely. Hold your hands like a pair of parentheses in front of your face. Now put them right up to your face. It looks like you’re scared of showing your face! Don’t be. And don’t be afraid of showing your sentences either, in all their glory. Use commas instead.
There are some great websites online to go to with your grammar questions (my favorite is Grammar Girl). However, sometimes you just need to know the rule, because you need to figure out how it applies to your sentence and writing in general. Commas are especially finicky.
In her bestseller Eats, Shoots and Leaves: The Zero Tolerance Approach to Punctuation, Lynne Truss, usually a strict prescriptivist when it comes to the rules of the punctuation marks, concedes that at times a take-it-or-leave-it attitude works with the comma. In regard to the serial comma, Truss writes that when “the stylistic reasons for its inclusion clearly [outweigh] the grammatical ones for taking it out,” it is OK to do what feels natural.