A concise summary of comma rules:
(the safest rule: Don't use a comma unless you have a reason.)

Use commas between items in a list...

  • using words: "Please pick up some rice, soy, and ginger at the store"
  • using phrases: "You can open cans, slice carrots, and bake bread with this magical appliance!."
  • using clauses: "I can't decide which classes to take, how I'm going to pay for it, or where I'm going to go."

Use commas in coordinate situations

Use commas after introductory elements in sentences

  • Nouns of address: "Dave, I'm sorry I can't do that."
  • Mild Interjections: "Well, I'm sorry I'm not going to be able to make it tonight."
  • Groups of prepositional phrases: "In the house without anything to do, he stayed up all night noding."
  • Participial words and phrases: "Regarding our research, please note our wonderful discovery."
  • Infinitive phrases: "To set the server up, just follow the instructions on page 802."
  • Subordinate clauses: "Soon after he saved his term paper, the power went out in the lab."
  • Absolutes: "The matter already decided, he concluded to go about his business."
  • Transition words and phrases: "First of all, I should explain my position."

Use commas to set off interrupting elements

  • Nouns of address: "I'm very sorry, Mrs. Davies, but your hard drive can't be recovered."
  • Appositives: "He bought a new car, a 1990 Honda Civic."
  • Contrasting elements: "The software, not the hardware, is the problem in this case."
  • Unnecessary descriptive phrases: "Iced tea, with or without lemon, goes great with chicken fried steak"
  • Unnecessary descriptive clauses: "The core dump, which can sometimes be useful, won't be any help to us here."
  • Absolutes: "Mark jumped at the sound of his alarm, its music blaring."
  • Parenthetical expressions: "We will ship everything at once (e.g., hardware, software, documentation)".

Use commas in standard places

  • Between parts of addresses in a sentence (but not zip codes): "1401 W. Airport Frwy., Euless, TX 76040"
  • Between and after parts of a date: "Sunday, October 8, 2001"
  • To set off direct quotations: "Thomas said, 'That's not true'."
  • In a business letter after the close: "Sincerely,"
  • In a personal letter after the opening and close: "Dear Frank, ....... Thanks,"
  • Between and after titles: "Mark Smith, Jr., is the only son of Mark Smith, Ph.D."
  • When reversing first and last names in a list: "Smith, Mark"
  • To change a statement to a question: "You're the boss here, right?"
  • In numbers: "123,456,789.00"


Source: Personal notes from High School English class.

Com"ma (?), n. [L. comma part of a sentence, comma, Gr. clause, fr. to cut off. Cf. Capon.]

1.

A character or point [,] marking the smallest divisions of a sentence, written or printed.

2. Mus.

A small interval (the difference beyween a major and minor half step), seldom used except by tuners.

Comma bacillus Physiol., a variety of bacillus shaped like a comma, found in the intestines of patients suffering from cholera. It is considered by some as having a special relation to the disease; -- called also cholera bacillus. -- Comma butterfly Zool., an American butterfly (Grapta comma), having a white comma-shaped marking on the under side of the wings.

 

© Webster 1913.

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