The comma is one of the most diminutive of punctuation marks, but an omitted or misplaced comma can cause considerable confusion. Here’s an example of how a comma can completely change the meaning of a sentence:
Let’s bake, Grandma!
Let’s bake Grandma!
A missing serial comma (the comma before the last item in a series) can also confuse the reader, though less dramatically. For example, suppose you’re emailing a friend to say that you invited four people over for dinner—your neighbors Joe and Tammy Smith and your friends Jan and Bob. You write:
We invited the Smiths, Jan and Bob.
Without the serial comma, this could mean that you invited only Jan and Bob, whose last name is Smith, over for dinner. By using the serial comma, you clarify what you really mean:
We invited the Smiths, Jan, and Bob.