Use a comma to separate independent clauses joined by coordinating conjunctions.
- I ordered spaghetti, and Juanita ordered a meatball sandwich.
- It was raining, so we decided not to go for a walk.
- She is usually cheerful, but she wasn’t smiling today.
Use a comma after introductory elements.
- When I told her my computer was broken, she loaned me her tablet.
- Not paying attention to where she was walking, Lisa lost her way.
- Smiling, he went upstairs to his room and closed the door.
If a short adverbial introductory element is clear, the comma is optional.
- In 2010 she bought her first car.
- After work I’m going bowling.
Vocatives must be set of by commas.
- Hi, Ralph.
- John, meet me out front in one hour.
- Can I borrow you car, Janet?
Use commas to separate items in a list, including the penultimate and last item.
- I bought sugar, instant coffee, and half-and-half.
Commas set off parenthetical insertions.
- The noise from the traffic, at times, can be annoying.
- Julie, for instance, does not eat food that contains gluten.
Use commas to separate coordinate adjectives.
- It’s a warm, pleasant day.
In dates, commas set off the year.
- They were married on July 1, 1950.
In addresses, commas set off the street address, the city, and the state.
These instructions are for standard writing. For instructions about mail, consult the United States Postal Service.
- She lives in San Diego, California.
- Mike is moving to Phoenix, Arizona, because he is tired of the cold weather in New York.
- He lives at 12345 State Street, San Diego, California.