The 5 Uses of a Comma

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The five uses of the comma are:

Separating the main elements of a sentence from each other
Setting off a parenthetical element from the rest of the sentence
Separating elements in a series
Setting off dialogs or quotations
Other uses of the comma

1. Separating the Main Elements of a Sentence from Each Other

Right or Wrong?
The students completed their math test on Monday, and the teachers handed in the grades on Thursday.

Right! The above sentence is composed of two independent clauses, each informing about two different people who did different things. A comma is needed before the coordinating conjunction, even though many English writers do not use a comma in such a case.

2. Setting off a Parenthetical Element from the Rest of the Sentence

Right or wrong?
Last week's test says my best friend was the hardest ever.
It Depends! In the above example sentence my best friend is probably not a part of the main sentence Last week's test was the hardest ever and should be separated with commas. Without commas, the reader may get confused and think that the test "says" that the friend was the hardest, while it is the test that was the hardest. Says my best friend serves as a comment, known as a parenthetical element. The correct punctuation would therefore be:
Last week's test, says my best friend, was the hardest ever.

3. Separating Elements in a Series

Right or wrong?
Dan's lifelong project is to be able to speak American English French German and Mandarin Chinese.

Wrong! In the above example sentence the language names come in a series whose elements should be separated with commas. In addition, not having a comma between "American" and "English" may make readers think that there is an independent language called "American," whereas it is considered only as an English language variety; or conclude that there is a French variety of German. The correct punctuation would therefore be:
Dan's lifelong project is to be able to speak American English, French, German, and Mandarin Chinese.

4. Setting off Dialogs or Quotations

1. A quote is a text that brings the exact words of a speaker in direct discourse. A text that consists only of dialogue (plays, novels etc.) is punctuated according to regular punctuation rules. However, in a text that combines both dialogue and non-dialogue text, the quotations are separated from introductory words (e.g., said, stated, explained, claimed) with quotation marks, commas and other punctuation marks.

2. The punctuation mark that comes before the quote is left outside the opening quotation mark. The punctuation mark that comes after the quote is put inside the closing quotation marks. The quote itself starts with a capital letter. See the following examples for different positions of the introductory words.
The singer Madonna said, "We are living in a material world."
[introductory words before the quote]

"We are living in a material world," said the singer Madonna.
[introductory words after the quote]

"We are living," said the singer Madonna, "in a material world."
[introductory words within the quote]

The singer Madonna said, "We are living in a material world," and left the stage.
[introductory and non quote words before and after the quote]

5. Other Uses of the Comma

A. Indicating Omitted and Repeated Words

1. A general stylistic convention in writing is that the more you can say in less words, the more elegant and polished the writing is. Elliptical constructions which omit words are one way of achieving such brevity in writing.

2. In the following examples, no comma is needed to indicate omitted words:

Madonna was sure that she would get an Oscar.
Madonna was sure ___ she would get an Oscar.

[Omission of that in object relative clauses]

Madonna looked angrily at Michael Jackson, and he at her.

[ellipsis]

Madonna is as short as I am.
Madonna is as short as I.

[ellipsis]

3. In the following examples, a comma is needed to indicate omitted words. The sentence may be ungrammatical without it.
Madonna's first album sold only 2,000 copies but her second, 2,000,000.

[Omission of album sold, comma instead]

In spring Madonna's fans sent her 1,000 letters; in summer, 3,000; and in the fall, none.

[ellipsis of they sent her, notice the semi-colons dividing between the three independent clauses that contain commas]

4. Use commas to separate words repeated within a sentence to avoid confusion.

Whatever that is here that smells, smells just awful!
What she does, she does well.
She came in, in tears.

B. Commas with Dates

1. Use a comma to separate the date from the year, when written in American style.
December 30, 1975 [12.30.75 - US style]

2. Do not use a comma to separate any element of the date, when written in British style (also common around the world and in the U.S. army.)
30th of December 1975 [30.12.75 - UK style]

Note: As confusion may arise from having two styles of dates, you had better not write dates using numbers alone. Instead, write the month's name as a word and the date and year as numbers.

3. Use a comma to separate the day from the date.
Tuesday, December 30 [12.30.75 - US date]

4. In a full sentence, use a comma on both sides of the year in a full date.
Many people were waiting on December 30, 1975, to celebrate the birth of the writer of this punctuation guide.

5. Do not use a comma when writing only two date elements, namely, the month and the year, the month and the day, or the season and the year.

Madonna's concert in August 1983 is well remembered by all her fans.
Madonna's concert on August 30 is well remembered by all her fans.
In summer 1983 Madonna gave a memorable concert.

C. Commas with Numbers

1. Use a comma as a thousands separator after every three digits in a number, counting from right to left.
87,950 people arrived to demonstrate in London against the government's decision. 1,850,400 signatures were collected nationwide in support of the protest.

(Note: In some countries a space is used instead of a comma, e.g. 87 9500)

2. A comma is optional with most four-digit numbers. Whatever option you choose, remember to be consistent in your writing.

This car costs 8,500 $. or This car costs 8500 $.
He drove for 2,250 miles. or He drove for 2250 miles.

3. Never use a comma in a four-digit year. Use a comma if the year has more than 4 digits.
In 1992 German zoologists discovered rare animal fossils from 35,000 BC.

4. Never use a comma in an address of four digits or more.
Beverly Hills 90210 was a very popular T.V. series.

5. Use a comma to separate related measurements written as words.
My son is five feet, four inches tall.

6. Use a comma to separate a scene from an act in a play.
act II, scene vi; or act 2, scene 4

7. Never use a comma in a page number of four digits or more.
For more information, see page 1378.

8. Use a comma to separate references to a page and line.
For more information, see page 1378, line 30.

9. Use a comma to separate two numbers that lie next to each other in a text.
In 1994, 3 of Madonna's songs reached the top ten in the music charts.

D. Commas with Names, Places, Addresses and Correspondences

1. Use a comma to separate people's names and their academic degrees. Use a comma after the degree if other words follow it.
Dan Smith, MD, will speak after Rosanne Smith, PhD.

2. Do not use a comma when an indicator of birth order or succession follows a name.
Marthin Luther King Jr. Henry VIII.

3. Use a comma if you write a person's last name before the first name.
Smith, Dan, MD, will speak second.

4. Use a comma to separate between two place names in sequence, e.g. city and county/state/country. Add another comma after the place name if more words follow.
Dan Smith, MD, is coming from Houston, Texas, to speak at the conference.

5. When a complete address is part of a sentence, use a comma to separate all the items, except the county/state/country and the zip code.
Dan wrote to Clara Clausowitz, 1001 Rule Road, Commaville, England ETS432, for more information about comma usage in English.

6. Use a comma for the opening of an informal letter. Some instructors suggest a colon in formal business letters.
Dear Dave, [friendly, informal] Dear Mrs. Clausowitz: [business, formal]

7. Use a comma for the closing words of any letter, and a point for the closing sentence that precedes it.
Hope to hear from you soon. [closing sentence]
Yours, or Love, [informal closing words]
sincerely, or Best regards, [formal closing word