comma (,)
Definition - The name for the , punctuation mark.


1. Use a comma after an independent clause when you use coordinating conjunctions such as and, but, and for to link it to a second independent clause.

I am going to Toronto, and I intend to stay there.

2. Don't connect two independent clauses with just a comma.

I am going to Toronto, I intend to stay there. (bad)

3. Use a semicolon to connect two independent clauses.

I am going to Toronto; I intend to stay there. (good)

4. Use a comma to separate adjectives if the word and could be inserted between them.

He is a mean, healthy man.

5. Use a comma to separate questions from preceding statements.

I can eat it, can't I?

6. When writing full dates, enclose the year in commas.

I bought a banana on September 12, 1990, at 9:15 AM.

7. But if the day is omitted, leave out the comma.

I bought my house in September 2001.

8. Use a comma before or on both sides of the name or title of a directly addressed person.

No, Doctor, I won't.

9. When writing the name of a city followed by its country, set off the country with commas.

I lived in Quebec City, Canada, for two years.

10. Use commas to set off expressions that interrupt a sentence's flow.

I am, as you might guess, very happy about this.

11. If a sentence starts with a subordinate clause, use a comma after the subordinate clause.

If you aren't happy with this, let us know.

12. If a sentence starts with a independent clause, don't use a comma.

Let us know if you aren't happy with this.

13. When the sentence begins with a phrase, use a comma if there are four or more words. If fewer than four, the comma is optional.

To succeed in this task, you must never give up.
To win you must never give up. (comma optional)

14. If there are two verbs and both have their respective subjects, insert a comma.

He ate way too fast, but he never got a stomach ache.

15. However, if a second subject does not appear before the second verb, don't use a comma.

He ate way too fast but never got a stomach ache.

16. Use a comma to separate contrasting parts of a sentence.

That's MY girlfriend, not yours.

17. Use commas when you start a sentence with introductory words like well, now, no, or yes.

No, I don't want any bananas.

18. Use commas for direct quotations that are three or fewer lines.

"How," I asked, "do you want to do it?"
Note: If the quote has more than three lines, introduce it with a colon, then leave a blank line above and below the quoted material.

19. Enclose words like therefore and however in commas if they are interrupting the sentence's flow.

I would not, however, like to stay in Toronto.

20. Use commas to surround degrees or titles used with names, but not around Jr. and Sr. or I and II.

George Bush Jr.
Charles Windsor, Prince of Wales, says "Hello."

21. Use a comma before introductory terms such as i.e., namely, or e.g. when they are followed by a series of items.

You could be asked to do many things, e.g. drink water, eat candies, and tell tales.
Note: Putting a comma after the introductory term is optional.

Etymology -
The word comma derives via Latin from the Greek komma, clause in a sentence (literally "a piece that is cut off"). It originally denoted a part of a sentence. Eventually, the sense was transferred to the punctuation mark that identified the part of the sentence. Purportedly, commas were invented by Aristophanes of Byzantium.

Oxford English Dictionary -
Its first citation in its punctuation-mark sense is from 1530:
"With suche [point] as the Latins call comma thus made (:), or virgula thus made (,).'
(Jehan Palsgrave, Lesclarcissement de la langue françoyse, 39 )

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