Punctuation Rules

The Comma


A comma does three things:

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.

What happens if the list items have commas in them?

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.

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

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

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.

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