The comma has lots of rules. I'll just cover three of the most important ones here.

Visual summary

Rule 1: When you join two full sentences with the word and, put a comma before the and.

Example:

Our new computers will be fast, and they will arrive in less than a week.

There's a good reason for that comma. Without it, as we're reading, we'll probably think the word and is going to join the word before it with another word like it:

Our new computers will be fast and cheap.

We're then surprised when the word and is joining an entire sentence to another entire sentence. The comma prevents that misreading.

Rule 2: Separate all items in a simple list with commas.

Example:

There are three reasons our new computers will be popular: speed, cheapness, and small size.

Notice the comma after the word "cheapness."

We've all had someone tell us to leave out the comma before the and. And we've all had someone tell us to put it in. The tendency today is to put it in. That's good. There will never be any misreading with it; there is sometimes misreading without it. What's the harm of saying, "I've ended an item (comma), I've ended an item (comma), and I've ended an item (period)"?

However, you need to know that a few style guides still say to leave out the final comma for a simple series. But the tendency today is definitely for putting it in. I always use it.

Rule 3: Use a comma after most introductory material in a sentence.

Example (the introductory material is in red):

After we get our new computers, they will be popular.

What if this is the introductory material?

After this week, we'll all have new computers.

That comma is optional. The reason the comma is optional in the second example is that the introductory material passes both of these tests:

1. It's short (nobody defines short).
2. It's a phrase (meaning that there's no verb in it).

It's still a good idea to put it in. Why not show your reader the structure of your sentence?

Frequently asked questions about the the comma

What's a common misuse of the comma?

Where do commas go in relation to quotation marks?

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Copyright 2002 by Edward P. Bailey
(all rights reserved)