Among the many fine gifts I received yesterday was a T-shirt bearing this message:

It came a semester late, but it’s a perfect illustration of my answer to this question, posted May 10:

“If I say, ‘Hello Diane,’ do I need a comma?”

The answer was yes, and the T-shirt (given by a real-life grandma, who may have a vested interest in getting it right) shows why. In Let’s eat, Grandma — the second version, with comma — the speaker is directly addressing Grandma and suggesting that it’s mealtime. In Let’s eat Grandma, without comma, the speaker is suggesting to someone else that . . . well, it’s not very pleasant for Grandma.

That’s just one way commas save lives.

Some years ago, I attended a holiday concert that included a song called “Throw the Yule Log on Uncle John,” attributed to P.D.Q. Bach. (If you don’t know his work, look it up.) Does the title need a comma? Think about it. If you decide it does — and Uncle John hopes you do — On needs to be capitalized, since it is used as an adverb rather than a preposition.

Don’t feel bad if you’re confused. For decades I heard a popular Christmas carol as “God Rest Ye, Merry Gentlemen.” Imagine my surprise when I learned it was really “God Rest Ye Merry, Gentlemen.” What’s the difference? In the first, the gentlemen were already merry; the correct version expresses a wish that God will make them so, and that’s the message intended.

Commas save sentences. They matter. Use them wisely.