Mastering the Art of Commas: When to Use Them in Your Writing

Commas help readers see how parts of a sentence fit together. Use them to separate items in a list, set off some introductory material and mark extra information. But a comma cannot usually join two complete sentences by itself, and it should not split a subject from its verb.

Separate items and introductory material

Use commas between three or more items in a list: We packed shirts, snacks and a map. A comma before and in the final pair—the serial comma—is optional in many styles. Use it if it makes the list clearer: We invited my cousins, Sam, and Lee could be read in more than one way; a rewrite may be clearer still.

Put a comma after an introductory dependent clause: When the meeting ended, we went for lunch. A comma is also common after a longer introductory phrase: After a long walk through the city, we stopped for coffee. With a very short phrase, the comma may be optional: In the morning we exercise and In the morning, we exercise are both acceptable.

Join clauses without creating a comma splice

When two complete sentences are joined by and, but, or, so, or yet, put a comma before the joining word: The train was delayed, so we called the hotel. Both sides could stand as sentences: The train was delayed. We called the hotel.

Do not use a comma alone to join two complete sentences. The train was delayed, we called the hotel is a comma splice. Use a full stop, a semicolon, or a comma plus a suitable joining word instead.

If the second part is not a complete sentence, a comma is often unnecessary: The train was delayed and arrived after midnight. The same subject applies to both verbs, so and arrived after midnight is not a complete sentence on its own.

Mark extra information—and keep essential details

Use commas around information that can be removed without changing which person or thing you mean: Maya, who works at the library, recommended this book. The reader already knows which Maya is meant.

Do not put commas around information needed to identify someone or something: The employee who speaks Spanish can help you. Without the identifying clause, it is unclear which employee you mean. In American English, this distinction often appears as who for extra information and that for essential information, but who can also introduce an essential clause.

Commas also separate a person’s name when speaking directly to them: Could you check this, Alex? Without the comma, the sentence may be harder to read.

Avoid commas that break sentence structure

Do not place a comma between a subject and its verb, even if the subject is long: The book you lent me is on the desk, not The book you lent me, is on the desk. Read the sentence aloud if you are unsure: pause only where the sentence’s structure or meaning calls for it, not simply where you take a breath.