There are several points to remember here. Be certain that you truly have two independent clauses, and not just a compound subject or verb. Also, make sure to include both a comma and a coordinating conjunction (and, but, for, nor, or, so, yet). Leaving out the coordinating conjunction creates an error known as a comma splice (see Chapter 54).

Use a comma after an introductory word group. Introductory word groups are descriptive phrases or clauses that open a sentence. Separate these introductions from the main part of the sentence with a comma.

Within two years of getting a degree in journalism, Ishan was writing for the Wall Street Journal.

For very brief introductory phrases, the comma may be omitted, but it is not wrong to leave it in.

After college I plan to join the Marines.

After college, I plan to join the Marines.

Use commas with common connective words and phrases. These would include items such as the following: however; therefore; consequently; finally; furthermore; nonetheless; specifically; as a result; in addition; for instance; in fact; on the other hand; that is. If a transitional word or phrase opens a sentence, it is usually followed by a comma.

Furthermore, medical reports suggest that trans fats lower the amount of good cholesterol found in the body.

On the other hand, studies of cholesterol have been notoriously controversial.

When used within a sentence, expressions such as however and for example should be set off by a pair of commas.

Big payrolls mean success in professional sports. In baseball, for example, teams from New York and Boston are almost always competitive. There are, however, notable exceptions.

Be especially careful with punctuation around however and therefore. A common error is to place commas around these connective words to link a pair of related sentences. This move produces an error called a comma splice (see Chapter 54 for more details). Here’s what that error looks like:

To correct this type of comma splice, you can place a semicolon before however or create two separate sentences:

In baseball, teams with big payrolls are almost always competitive; however, there are notable exceptions.

In baseball, teams with big payrolls are almost always competitive. However, there are notable exceptions.

Put commas around nonrestrictive (that is, nonessential) elements. You’ll know that a word or phrase is functioning as a nonrestrictive modifier if you can remove it from the sentence without obscuring the overall meaning of the sentence.

Cicero, ancient Rome’s greatest orator and lawyer, was a self-made man.

Cicero was a self-made man.

The second sentence is less informative but still makes sense. See also the guideline, “Do not use commas to set off restrictive elements.”

Use commas to separate items in a series. Commas are necessary when you have three or more items in a series.

American highways were once ruled by powerful muscle cars such as GTOs, Road Runners, and Gran Sports.

Do not use commas to separate compound verbs. Don’t confuse a true compound sentence (which has two independent clauses) with a sentence that simply has two verbs.

They rumbled through city streets is an independent clause, but and smoked down drag strips is not, because it doesn’t have its own subject. To join two verbs that share a common subject (in this case, they), all you need is and. When you have three or more verbs, however, treat them as items in a series and do separate them with commas. Compare the following examples:

Do not use a comma between subject and verb. Perhaps it’s obvious why such commas don’t work when you notice one in a short sentence.

When a subject gets long and complicated, however, you might be more tempted to insert the comma. It would still be both unnecessary and wrong. The commas in the following sentences should be omitted.

Do not use commas to set off restrictive elements. Phrases you cannot remove from a sentence without significantly altering meaning are called restrictive or essential. They are modifiers that provide information needed to understand the subject.

Only nations that recognize a right to free speech and free press should be eligible for seats on international human rights commissions.

Students who have a perfect attendance record will earn three points for class participation.

Delete the blue phrases in the above examples and you are left with sentences that are vague or confusing. Put commas around the phrases and you create the false impression that they could be removed.