The comma is probably the most versatile of all punctuation marks. This means you as a writer can use your judgment in many cases as to whether you need a comma or not. It also means that the possible errors involving commas are many. Commas are necessary some of the time, but careless writers often place a comma in a sentence where it is simply not needed.
Commas are used to separate two independent clauses joined by a conjunction like “but,” “and,” and “or.”
Example
The advertising department is effective, but don’t expect miracles in this business climate.
Commas are not used simply to join two independent clauses. This is known as the comma splice error, and the way to correct it is to insert a conjunction after the comma.
Examples
The advertising department is effective, the sales department needs to produce more results.
The advertising department is effective, but the sales department needs to produce more results.
Commas are used for introductory phrases and to offset clauses that are not essential to the sentence. If the meaning would remain intact without the phrase, it is considered nonessential.
Examples
After the summary of this year’s sales, the sales department had good reason to celebrate.
The sales department, last year’s winner of the most productive award, celebrated their stellar sales success this year.
The sales department celebrated their stellar sales success this year.
Commas are used to offset words that help create unity across a sentence like “however” and “therefore.”
Examples
The sales department discovered, however, that the forecast for next year is challenging.
However, the sales department discovered that the forecast for next year is challenging.
Commas are often used to separate more than one adjective modifying a noun.
Example
The sales department discovered the troublesome, challenging forecast for next year.
Commas are used to separate addresses, dates, and titles; they are also used in dialogue sequences.
Examples
John is from Ancud, Chile.
Katy was born on August 2, 2002.
Examples
Mackenzie McLean, D. V., is an excellent veterinarian.
Lisa said, “When writing, omit needless words.”
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