Punctuation
Comma

Comma

Items in a series

Each item in a series of more than two needs to be separated by a comma. Even though some sources say that the last comma is optional unless it is required for clarity, we recommend that you always place a comma after the second last item in the series.

  My favorite pies are lemon, blueberry, rhubarb and strawberry.

Are there three or four pies? Is the last pie made with rhubarb and strawberries? If it is, there are only three pies, so the sentence should be written in this way.

  My favorite pies are lemon, blueberry, and rhubarb and strawberry.

If there are definitely four pies, the sentence should be written in this way.

  My favorite pies are lemon, blueberry, rhubarb, and strawberry.

With the extra comma, we know that there are four pies. There is no lack of clarity in that sentence.

Subordinate clause

A subordinate clause at the beginning of a sentence needs to be set off by a comma.

Though the runner was behind, he still expected to win the race.

Long prepositional phrase

A long prepositional phrase at the beginning of a sentence needs to be set off by a comma.

Into the long and dreary night with the wind howling around him, the trapper trudged on.

Note: Do not use a comma after a short prepositional phrase, unless it is needed for clarity.

In the spring we open the windows to allow the air to flow through the house.

Addresses

The various items that make up an address in a sentence need to be separated by commas.

Please address the envelope to Mr. John Walker, 376 Howard Street, Halifax, Nova Scotia H0H 0H0.

Note: Do not place a comma between the province and the postal code or between the state and the zip code. Also, do not insert a hyphen in the postal code

If you have a city and a province or state in a sentence, you place a comma after the city and another one after the province or state.

Our son was born in Guelph, Ontario, on March 12, 1981.

Dates

The items in a date that includes the month, day, and year need to be separated by commas.

It was on January 12, 2009, that Haiti experienced a major earthquake.

Note: Do not place a comma between a date that includes just the month and the year.

It was in January 2009 that Haiti experienced a major earthquake.

Negative expression

When not is used in a sentence to create a negative expression, commas are used to set off the expression.

Orange juice, not apple juice, is my favorite breakfast drink.

Parenthetical expressions

These are expressions that tend to interrupt the natural flow of the sentence, so they need to be set off by commas. If you need to pause after the parenthetical expression, then a comma is required.

It was Kim, by the way, who told me about the accident.
In my opinion, this problem should be resolved by mediation.
The mayor, as well as many councillors, attended the ceremony.
Everyone at the hospital, including the doctors and the nurses, prayed for the child's recovery.

If the parenthetical expression does not interrupt the natural flow of the sentence, it does not need to be set off by commas. If you do not need to pause after the parenthetical expression, then a comma is not required.

It was indeed Kim who told me about the accident.

Parenthetical expressions can be one word or more than one word. Here is list of some common parenthetical expressions.

accordingly, consequently, for example, for instance, hence, however, in addition, in fact, moreover, nevertheless, otherwise, therefore

If a parenthetical expression is used after a semicolon, a comma is required.

You must spend more time studying; otherwise, you will fail your courses.
The students were late for their class; consequently, they were given detentions.

Appositives

An appositive is a word that is placed beside another word to provide more information about the first word.

Thomas, the artist, hung his watercolors in the art gallery.

Note: If you use the phrase the artist Thomas, there is no comma, because we need Thomas to identify the artist.

The artist Thomas hung his watercolors in the art gallery.

The appositive can also appear at the beginning of the sentence.

The son of an Italian immigrant, the CEO began his own career at Proctor & Gamble and stayed there for a decade.

Salutations in social letters

A comma is used after the salutation in a social letter. However, a colon is used after the salutation in a business letter.

Dear Aunt Jane,
My dearest cousin,

Direct quotations

A comma is used to separate the speaker from the quotation.

"According to our records," said the accountant, "you will need to pay more income tax this year."

Note: The comma is placed before the quotation mark.

Certain words at the beginning of a sentence

Words such as yes or no at the beginning of a sentence are set off by a comma.

Yes, I would like to go the concert with you.
No, I will not participate in the debate.

Prevent misunderstanding

Sometimes, for clarity, a comma must be used in a sentence.

As he watched, the ladder began to sway in the wind.

Without the comma we tend to read As he watched the ladder without pausing. Then we realize that the sentence does not make sense unless we pause (insert a comma) after watched.

Direct address of a person

When a person is addressed directly in a sentence, the person's name is set off by a comma or commas, depending on its location in the sentence.

If I were you, Peter, I would not accept that offer.
Peter, I would not accept that offer if I were you.

Restrictive and non-restrictive clauses

Restrictive clause

A restrictive clause is a clause that is necessary to identify the noun to which it refers; therefore, it is not set off by commas from the rest of the sentence.

The man wished to remain anonymous.

When we read this sentence, we want to know "Which man wished to remain anonymous?"

The man who helped the stranded motorist wished to remain anonymous.

We need the clause who helped the stranded motorist to identify this man from many others and to tell us who wished to remain anonymous. Therefore, it is a restrictive (necessary) clause.

By using the restrictive clause, we now know which man wished to remain anonymous (although we still don't know his name).

Therefore, because the clause is necessary to identify the subject, it is not set off by commas.

Non-restrictive clause

A non-restrictive clause is a clause that is not necessary to identify the noun to which it refers, so it is set off by commas from the rest of the sentence.

Mr. Smith, who was the principal of the school, addressed the students in the auditorium.

We do not need the clause who was the principal of the school in the sentence to identify Mr. Smith because we already know his name. If we omit the non-restrictive clause, we have Mr. Smith addressed the students in the auditorium.

Because this clause is not needed to identify who addressed the students, it is a non-restrictive (unnecessary) clause; therefore, it is set off by commas.

Compound sentence

A comma is used to separate two principal clauses that are joined by a coordinate conjunction.

Mark was late for class as usual, but he did not get a detention.

Equal adjectives

If two adjectives are of equal grammatical value, a comma should be placed between them.

Some counterfeit products are filled with dirty, dangerous materials.

Both these words modify materials.

Hint: The adjectives are of equal grammatical value if

  • you can place the word and between the two adjectives, and the sentence still makes sense, or
  • you can change the order of the adjectives, and the sentence still makes sense.
Some counterfeit products are filled with dirty and dangerous materials.
Some counterfeit products are filled with dangerous and dirty materials.

Because the adjectives have passed the and test, a comma must be inserted between them.

Numbers

A comma is used to separate the numerals in a number that has more than four numerals before the decimal point. The comma is placed to the left of the decimal point after every three numerals.

420,982
598,626,982

Caution

Do not place a comma in a sentence unless there is a punctuation rule for it. Otherwise, the sentence is considered grammatically incorrect.

  The man that buys what he does not need, steals from himself.

There is no punctuation rule for placing a comma in the sentence. Therefore, the sentence is punctuated incorrectly.

  The man that buys what he does not need steals from himself.

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