What is a comma?
You’ve probably heard a lot of things about the comma and may have questions about when to use a comma. A comma (,) signifies a short pause in a sentence. It can also divide clauses (“parts of a sentence”) or items in a list. It is often used to create division or to improve the clarity of a sentence.
When to use a comma
1. To signal a pause
In writing, commas usually signal a pause that would be heard if the sentence were read aloud. It’s a short, soft pause, as opposed to the longer pause signified by a period.
2. To separate adjectives
Commas separate adjectives when the order of the adjectives doesn’t affect the meaning. For example: “She gave him a soft, comfortable blanket.” The order of the adjectives soft and comfortable could be reversed . . . so, they’re separated by a comma. However, in “delicious chocolate cake,” the word chocolate is a direct modifier of cake. So, the order of delicious and chocolate shouldn’t be flipped. (In this case, the comma isn’t needed.)
3. In nonrestrictive (nonessential) clauses
A comma can also separate nonessential words from the essential parts of a sentence. For example, “Patty came to visit,” doesn’t need a comma. But, “Patty, my second cousin, came to visit,” does need commas to for the nonrestrictive clause (not necessary for the sentence to be understandable): my second cousin. Overall, these nonrestrictive clauses basically just add extra information to a sentence.
When you use as well as in a nonrestrictive clause, the same rules apply: “Deborah, as well as her assistant, decided to attend the convention.” However, when there is a restrictive clause (which also modifies a sentence, but it adds essential information), you shouldn’t offset it with commas, even when it contains as well as. An example is “The employee discount policy applies to full-time workers as well as contractors.” If you remove as well as contractors, the sentence loses some essential information.
4. To separate independent clauses
Commas are also used to separate independent clauses (“phrases that can stand on their own”) when a conjunction (like and) is used, as in the compound sentence: “Mark went to the store, and he bought eggs.”
5. To set off an introductory phrase
Commas set off introductory participle phrases as in: “Reading over her notes, Julie realized she missed an important detail.”
6. In numbers
You can also use commas to divide sets of numbers. For example, in numbers over 1,000, the comma separates sets of three digits at a time. For example, in 1,000,000, there are two commas (one for every three decimal places).
In a street address, commas divide each piece of information. For example, the address of the Metropolitan Museum of Art in New York is 1000 Fifth Avenue, New York, New York, 10038.
What is the Oxford comma?
Commas can separate items in a list. In a list of three or more items, the last comma is called the Oxford comma (or the serial comma). For example in “He bought eggs, milk, and bread,” there’s a comma between each item listed (the comma before and is the oxford comma). Not all style guides agree on whether to use the Oxford comma.
In some cases, you can leave the Oxford comma out without changing the meaning of the sentence. If you delete it from the previous example, it still has the same meaning: “He bought eggs, milk and bread.”
In other cases, the Oxford comma can be necessary. For example, in the sentence “I love my pets, chocolate, and pizza,” the Oxford comma makes it clear that all three items are separate. This one could be confusing if the Oxford comma were left out: “I love my pets, chocolate and pizza,” might mean that the speaker’s pets are named Chocolate and Pizza.
Who invented the comma?
The modern comma descended directly from Italian printer Aldus Manutius. (He’s also responsible for italics and the semicolon!) In the late 1400s, when Manutius was working, a slash mark (/, also called a virgule) denoted a pause in speech. (Virgule is still the word for comma in French.) Manutius made the slash lower in relation to the line of text and curved it slightly. In the 1500s, this new mark acquired the old Greek name comma. The word comma literally meant “a piece cut off,” which is from the Greek word koptein meaning “to cut off.”
Other than the period, the comma is the most common punctuation mark in English. What a confusing little guy.