“When should I use a comma?” is the question about writing I am asked more than any other.
Below are the rules. But experts disagree on many of them, and good writers ignore some of the rules. If you are a student, ask your teacher what she expects. Language is always changing, and that includes rules for writing language. In general, as American English has become more informal, fewer commas are used today than in the past.
- The rule: In a series of three (red, white, and blue), use two commas. The practice: Many people skip the comma before the word “and.” They think, if I don’t need a comma between “white and blue,” why do I need one for “red, white and blue”?
- The rule: In a compound sentence, a comma goes after the first clause. (I like snow, so I like winter sports.) In practice: If the clauses are short, most good writers skip the comma if the meaning is clear. If the clauses are long, they use the comma.
- The rule: Use a comma after a stand-alone adverb which starts the sentence. (First, let me eat. Then, we can talk.) The practice: Sometimes the comma is used, but many times it is not if the meaning is clear.
- The rule: When a dependent clause starts a sentence, end that clause with a comma before writing the independent clause. (The previous sentence is an example of that.) The practice: Good writers follow this rule. What if the first clause is the independent clause? No comma is needed. Students make lots of mistakes with this rule, especially when using the word “because.”
- The rule: When you start a sentence with “because,” you cannot put a period at the end of that clause. Instead, you must end that clause with a comma and continue the sentence with an independent clause. The practice: Teachers tell students they can’t start sentences with “because” to avoid students’ writing fragments. Of course, you can start a sentence with almost any word, including “because,” if you use correct sentence structure and punctuation.
- The rule: Appositives require commas before and after. (My teacher, Mrs. Smith, gives lots of homework.) If an appositive ends the sentence, then the “after” comma becomes a period. The practice: Commas are often not used with appositives.
- The rule: The identity of the person speaking a direct quote needs to be set off with a comma. (Mom said, “Eat your dinner.” “Eat your dinner,” Mom said.) If the spoken words end with a question mark or exclamation point, then the comma is not used. (“Look!” said Mom. “Where?” I asked.) The practice: Most good writers use this rule. If the quote is indirect, commas might or might not be needed. (Eat your dinner, my mother said. I said I would.)
- The rule: Between cities, states and countries commas are needed, but not between states and zip codes. The practice: This rule is used.
- The rule: Between days of the week, dates, and years, commas are needed. (My vacation stopped on Saturday, August 13, 2016, when I returned home.) Notice that if the date does not end the sentence, a comma is required after the date or year. The practice: This rule is followed by good writers.
- The rule: If just a month and year are used, no comma is required. (He graduated in May 2016.) The practice: This rule is generally used, but some grammar books require a comma in the last sentence. (He graduated in May, 2016).
- The rule: If “yes” or “no” begin a sentence, those words are followed by a comma. (Yes, I can hear you.) The practice: This rule is followed.
- The rule: When speaking directly to someone, a comma is used before or after the person’s name. (Lou, come here.) The practice: This rule is followed.
- The rule: To offset a negative phrase, commas are used before and after. (I saw Annushka, not Sei, at the movies.) The practice: This rule is followed.
- The rule: For house numbers, no comma is needed. But for other numbers of a thousand or more, commas are needed to separate every three numerals beginning from the right or decimal point. The practice: This rule is followed.
If you are using a grammar book as a reference, check the date. Older versions require more commas. If you are using a source from outside the US (English booklets prepared by a foreign company, for example), more commas will be required. If you are a high school or college student, ask your teacher which style book he or she will use to grade your work and follow that style book’s rules.