The comma (,) sets off or separates words or groups of words within sentences. Commas are the most common punctuation mark inside a sentence. However, the trend today is to use them only when absolutely necessary, or when omitting the comma would cause confusion.

Use a comma after a long introductory phrase or clause: "After working all day at the office, I went home for dinner." If the introductory material is short, forget the comma: "After work I went home for dinner."

Use a comma if the sentence would be confusing without it, as in: “The day before, I borrowed my boss's calculator."

Use a comma to separate elements in a series, including numbers in a list: "I enjoy drinking orange juice, tea, milk, and coffee." You also use it with numbers: “5, 7, and 9.” (There are some style guides that omit the comma before “and,” but you’ll notice that we like it. The important thing is to make sure that you are being consistent, and that you use what your organization is using.)

Use a comma to separate independent clauses that are joined by and, but, or, nor, for, or yet. "We shopped for three hours, but we didn't make a single purchase."

Use commas to set off nonessential elements in a sentence. Compare these two sentences:

• In this sentence: “At the podium stood a man wearing a green suit,” the phrase “wearing a green suit” is essential to identify which man.
• However, in this sentence: “At the podium stood Frank, wearing a green suit,” the phrase “wearing a green suit,” adds nonessential information about Frank.