Q: I notice that you put a comma between “Hi” and “Laura” in a personal reply to a grammar question from my wife. Is this a typo (in which case my apologies for pointing it out) or is this a correct usage (in which case I’m interested to learn why)?
A: I use commas before (or after or around) names used in direct address (that is, when you’re addressing somebody), as in “Hello, Laura,” or “Rodney, welcome,” or “Honey, I’m home!”
If the name is at the beginning of a sentence, you put a comma after it. If it’s at the end, you put the comma in front. And if the name is in the middle of a sentence, commas go in front and back.
This is a traditional rule of punctuation. Here are excerpts from a few style manuals:
Words Into Type (3d ed.): “Set off proper names and substantives used in direct address.” (P. 203)
The Chicago Manual of Style (15th ed.): “A comma follows names or words used in direct address and informal correspondence.” (P. 247)
Merriam-Webster’s Collegiate Dictionary (11th ed.): “[The comma] sets off words in direct address and mild interjections.” (P. 1605)
This is a longstanding convention, but many people don’t use a comma after “Hi,” probably because it’s so informal. I’m not bothered when people omit the comma, but I’ll continue to use it myself, because old style habits die hard.