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Commas are used to separate numerals or "digits" in large numbers. For numbers of four digits or more, place a comma before every third digit, counting from the right.

This printer costs $3,045.

Note: It is also acceptable to omit the comma in numbers with only four numerals or "digits."

Exceptions: Commas are not used in address numbers or in identification numbers.

Note: Spaces, not commas, are used in metric measurements. (This avoids confusion in those countries where commas are used as decimal points.)

14 267.9 hectares (U.S.A.)
or 14 267,9 hectares (European)

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Commas are used to set off an appositive, a noun or phrase that identifies the noun or pronoun it follows. (A restrictive appositive is essential to the basic meaning of the sentence; do not set it off with commas. See the second example below.)

Scott Erickson, a landscape designer, uses his laptop computer in the office and in the field. (nonrestrictive appositive)

Landscape designer Scott Erickson uses his laptop computer in the office and in the field. (restrictive appositive)

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Which and That: Use which to introduce nonrestrictive (unnecessary) clauses; use that to introduce restrictive (necessary) clauses. Doing so will help readers quickly distinguish essential information from nonessential information.

The system that we implemented in March 2002 was selected after a year-long study.

Note: The clause beginning with that is necessary to identify which system. No comma is used with this clause.

The new system, which was implemented in March, has already improved productivity 40%.

Note: The main clause tells the reader important information about the new system; the clause beginning with which gives additional - or nonessential information. Commas are needed to set off this unessential clause.

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Commas are used to set off nonrestrictive phrases and clauses used as modifiers. Nonrestrictive phrases or clauses are those that are not essential to the basic meaning of the sentence.

Roy, who is training to be a supervisor, is an asset to our service department. (nonrestrictive)

Good service at a reasonable rate, which sums up the department's philosophy, is the reason for the dealership's success. (nonrestrictive)

Note: The two clauses shown above in red are merely additional information; they are nonrestrictive (not required). If the clauses were left out of the sentences, the meaning of the sentences would remain clear.

Restrictive phrases or clauses - phrases or clauses that are needed in the sentence because they restrict or limit the meaning of the sentence - are not set off with commas.

Employees who are praised for new ideas are apt to be creative. (restrictive)

Companies that offer flexible hours usually have happier, more efficient workers. (restrictive)

Remember: Restrictive phrases are required in a sentence; nonrestrictive phrases are not required. Compare the following phrases:

The humorist Will Rogers was born in Oklahoma. (Will Rogers is required; do not use commas.)

Will Rogers, the humorist, was born in Oklahoma. (The humorist is not required; use commas.)

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Commas are used to set off the exact words of the speaker from the rest of the sentence.

"Nothing in fine print is ever good news," quipped Andy Rooney.

Do not use a comma before an indirect quotation. (The comma in red below should not be used.)

The computer technician said, that he would be here shortly. (misuse of a comma before an indirect quotation)

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