Punctuation Rules

The Comma

A comma does three things:

it sets off items in a list,
it separates a main clause from its free modifiers, and
(with a coordinate conjunction) it separates two main clauses.

Besides discussing these three rules, this page also contains a discussion of sentence fragments, comma splices, and run-ons

Coordinate Conjunctions: and, so, but, for, or

Examples

1a. A comma sets off items in a list.

On our farm, we grow wheat, corn, oates, soybeans, and alfalfa.
Normally, we order two burgers, two fries, two shakes, and one apple pie.

What happens if the list items have commas in them?

We sent copies of the memo to Walla Walla, Washington; Traverse City, Michigan; and New Brunswick, New York.

Notice that when there are commas within the list items, you place a semicolon between the list items.

1b. A comma separates a main clause from its free modifiers.

When we could stand it no longer, we called to find out when they were coming. (The comma sets off an initial-position subordinate clause.)

The athletes, marching in as a group, smiled excitedly as they looked up at the stadium full of spectators. (Two commas set of a verb cluster.)

Mary Anne knocked on the door three times, convinced, as she was, that they had to be inside. (The first comma sets of the terminal-position adjective cluster from the main clause. The second and third commas set of the embedded subordinate clause from the adjective cluster.)

1c. With a coordinate conjunction, a comma separates two main clauses.

Jill fetched a pail of water everyday, but Jack accompanied her only on Saturdays. (Notice that the coordinate conjunction separates two MAIN CLAUSES, not two nouns or two verbs. Neither does it separate the subject from the predicate.)

What happens when one of the main clauses has internal punctuation?

They drove to Ottawa that Tuesday morning; and, when they arrived at the courthouse, they nervously sought a parking place. (In this sentence, the subordinate clause in the second main clause is set off from its main clause with commas, so the two main clauses are separated from each other with a semicolon and a coordinate conjunction.)

What are sentence fragments, comma splices, and run-on sentences?

These three punctuation errors, usually considered major errors in student writing, result from the failure to recognize a main clause.

A sentence fragment is a group of words that is not a main clause, but it has been punctuated as though it were:

When I have finished my homework.

The above group of words has a capital and a period, so it is pretending to be a complete sentence, but it is only a subordinate clause. In dialogue, this fragment might be punctuated as a sentence if someone is portrayed as responding to a question like, "When can we go for a walk?" In other cases, it should be attached to a main clause and set off with a comma.

A comma splice consists of two main clauses which are not punctuated as such:

They drove to Ottawa, they looked for a parking place.

This apparent single sentence contains two main clauses attached to each other with nothing more than a comma. A comma just doesn't have enough power to "splice" or "join" two main clauses by itself. You need a coordinate conjunction, or you might turn one of the main clauses into a free modifier, like this: After driving to Ottawa, they looked for a parking place. A run-on sentence is a comma splice without the comma. Fix it the same way you would fix a comma splice.