A comma is used in the middle of, and never at the end of a sentence. This is done to make the sentence is complete, and also clearer, especially to separate items in a list. However, overusing commas can complicate a sentence, or render it meaningless.
A comma is important to avoid any possibility of misunderstanding a sentence as the following example shows:
While my father was eating my mother was drinking.
(Without the comma, it appears my father was eating my mother.)
While my father was eating, my mother was drinking.
A comma is used to separate:
words in a list.
- We saw monkeys, porcupines, lions, and tigers in the zoo.
- I met her parents, uncles, aunties, cousins, brothers, sisters, and pet dog.
more than one adjective that modifies a noun.
- The short, puny man was a comedian.
names and titles.
- Mr. Hillyholly, the General Manager.
A comma is used to separate:
parts of an address.
- 999, Downing Street, Kingston
cities and states / provinces, states / provinces and countries
items in dates, that is day of week from the date, and day of month from the year
- He is marrying on Tuesday, 1st April.
- The explosion happened on 1st January, 2000.
- It happened on September 11, 2001.
numbers that exceed three digits
- 2,000 / 20,000 / 200,000 / 2,000,000
A comma is used after:
the name of the person spoken to or directly addressed.
- Jane, let me show you how to do it.
exclamations and expresions from the rest of the sentence.
- Oh no, it can't be true.
- Well, all I can say is it's a waste of money.
greetings and closings in a formal letter
- Dear Princess Diana, / Yours sincerely / Yours truly
such words or phrases: of course, therefore, for exmaple, however, moreover, etc. to begin a sentence.
- Of course, you are always right.
- Moreover, the rent is high and the location is not perfect.
a phrase that begins a sentence.
- From the tunnel, it took the train another hour to reach our destination. (Prepositional phrase)
- Attracted by the brightly-coloured flowers on sale, we decided to have a closer look. (Paricipial phrase)
a subordinate clause.
- When they saw me, they pretended they didn't know me.
A comma is used to set off interrupting words, which can be:
- The cheetah, don't you know, is the fastest animal in the world.
- The man, who lives next door, is a circus clown. (Relative clause)
- Yesterday as I was jogging in the park, I saw what looked like a black baboon. (Subordinate clause)
a nonrestrictive clause.
- Traditional Indian dishes, which are my all-time favourites, are available in this restaurant.
- Her sister, a well-known eye surgeon, was admitted for eye surgery.
- An insomniac, my grandfather goes to bed when others get up to go to work.
a direct quotation.
- He said, "I believe the world is flat."
- "This town's history," he said, "goes back to Roman times."
A comma in a compound sentence
A comma is used where there are two independent clauses joined by a coordinating conjunction. The two clauses together form a compound sentence. The comma is used before the coordinating conjunction.