If you read my blog regularly, you know that I have a longstanding gripe about the way writing is taught: Textbooks and curriculums don’t build on what students already know.

Here’s an example. Below are two incomplete sentences. Finish them any way you like – that’s not the point. Here’s what I want you to think about: Which version indicates that Marilyn wasn’t invited to the party? Choose A or B.

A)  Everyone was invited to the party, but Marilyn

B)  Everyone was invited to the party but Marilyn

You choose B), right? You could tell that in version A), Marilyn was invited but probably wouldn’t be able to come:

Everyone was invited to the party, but Marilyn had other plans that evening.

Everyone was invited to the party but Marilyn and Dennis.

Here’s my point: You know more than you think you do!

Now let’s build a rule out of what you just did. Any time you use and or but, stop and think: Are you starting a new sentence? If so, use a comma before you start your new sentence. (It acts just like a period.)

Everyone was invited to the party, but Marilyn had other plans that evening. [A new sentence follows but]

I had a hot fudge sundae, and Joe had a milkshake. [A new sentence follows and]

I had a hot fudge sundae and a root beer. [There’s only one sentence]

You can download a free handout about commas at .