Here’s something that always seems to be a little confusing. Take a look at these two sentences and notice the absence of the comma in #2:
1. I stayed out all night partying with my sister, Lori.
2. I stayed out all night partying with my sister Lori.
Which is correct? Either could be, depending on how many sisters I have. In real life, I have three sisters, so the second sentence is correct.
Why is that? The answer has to do with essential and nonessential phrases. According to the Associated Press Stylebook 2006, an essential phrase is “a word or group of words critical to the reader’s understanding of what the author had in mind.” A nonessential phrase “provides more information about something” and “although the information may be helpful to the reader’s comprehension, the reader would not be misled if the information were not there.”
That said, here are a couple of rules:
RULE #1: Set off nonessential phrases with commas.
RULE #2: Do not use commas to set off an essential phrase from the rest of the sentence.
In this example, “Lori” is an essential phrase; since I have more than one sister, her name is critical if the reader is to know which sister kept me out partying all night. That’s why her name should not be set off with a comma, as in sentence #2.
The first sentence would be correct if I had only one sister, in which case Lori’s name would be nonessential; what other sister would I possibly be talking about?