One of the most common places in which there is confusion is over things like introductions, like the following:

• "John, this is my wife, Mary."
  In this case, "Mary" is an apposite phrase and must be set off with a comma.
  
  
• "John, this is Mary, my wife."
  In this case "my wife" is technically an apposite phrase, as you couldn't use this without a comma -- but it's clumsy. Presumably the speaker is explaining to John that Mary is his wife, but you could still use the sentence without the second phrase.
  
  
• "John, this is my sister, Joan."
  This is an apposite phrase because it suggests that the speaker only has one sister, whose name is Joan. The name provides more information, but it is still directly synonymous with "my sister." but....
  
  
• "John, this is my sister June."
  This is not an apposite phrase because it is suggesting that the speaker has more than one sister (perhaps Joan and June), and the speaker is therefore clarifying not only that this is his sister, but which sister it is. If you put a comma here, you would be implying that the speaker only has one sister, named June. Without the comma, you are indicating that the speaker may have several sisters and you must clarify which it is.

An apposite phrase is one that can often be dropped without damaging the structure of the sentence or badly obscuring the meaning. Thus, you can write: "British novelist Jane Maladroit..." and you must omit the comma, because you can't actually drop the words "Jane Maladroit" without badly obscuring the meaning of the sentence. There are many British novelists; there is only one Jane Maladroit. So, as with the "my sister June" example, you need to keep the name in the sentence to make the sentence clear.

However, you could write: "Jane Maladroit, British novelist..." as an apposite phrase (with the commas), because in this case, you are saying Jane Maladroit = a British novelist. It is an added bit of definition to the subject "Jane Maladroit," and while it's helpful, it could be removed without damaging the sentence too badly. You could write, "Jane Maladroit won the prize," and the reader would understand the sentence and come away with the information needed. But you couldn't say "British novelist won the prize" and give the same information.

So here's a quick and easy way to test whether something is an apposite phrase or not. Can you put in an "equal sign"?

• "My wife = Mary" - probably you only have one wife.
  
  
• "My sister = June" - works only if you have only one sister
  
  
• "Our sister site = LoadsofSisters.com" - works if you have one sister site.
  
  
• "Jane Maladroit = British author" - Jane is a British author.
  
  
• "John Smith's new book = Understanding the Comma" -- a bit "iffy" but it works if John Smith has only brought out one new book recently.

Here are some phrases where you can't put in an equal sign:

• "My sister > June"
  If you have more than one sister, then you are specifying which sister you are referring to; therefore, sister ≠ June.
  
  

  "British author > Jane Maladroit"

  There are many British authors, so you could not remove the information "Jane Maladroit" and keep the same meaning.
  
  

  "John Smith's book > Understanding the Comma"

  If John Smith has written many books, this clarifies which one you are referring to and is therefore essential.

Note that an apposite phrase is nonessential information, even if it is useful. Commas are a common way to set off nonessential information -- they tell the reader, "everything between these two bits of punctuation clarifies what went before." Keep in mind that this kind of phrase must be opened and closed with a comma -- "Jane Maladroit, British novelist, won the prize," not "Jane Maladroit, British novelist won the prize." You would say "My sister June went to the fair, while my sister Joan went to church," but if you have only one sister, you would say "My sister, Joan, went to church" (because you could remove the name "Joan" without destroying the meaning of the sentence).

Now, one more bit of confusion: You can sometimes avoid the whole comma issue by putting the apposite phrase before the subject:

• British novelist Jane Maladroit won the prize.
  
  
• American writer Moira Allen wrote a lengthy treatise on apposite phrases and had the nerve to call it "short."
  
  
• British writer Anne Nonymus often wished American writer Moira Allen would shut up already.

In these cases, the phrase stands as an adjective (which is, at rock bottom, what an apposite phrase is, and that was one). It can't always be done, but one way to test whether something is an apposite phrase and needs to be set off by commas is by testing to see if you can turn it around this way and retain the same meaning.

By the way, in these last three examples, one of the most common mistakes I see is to put in the commas that, one feels certain, must surely belong there because, after all, it's an apposite phrase, right? Wrong! The rule that applies to this variation is the same as the previous rule: What is put between commas must be something that can be removed without changing the meaning of the sentence.

Thus, it's vital to avoid the temptation to write "British novelist, Jane Maladroit, won the prize. The apposite portion of the phrase is the first part ("British novelist"), not "Jane Maladroit." The sure way to tell is to put in the commas, then try removing that portion of the sentence that you have "becomma'd," and see if it still says the same thing or even makes sense. If it doesn't, take the commas out.

And now I, Moira Allen, American author [double apposite phrase!] have confused you, dear reader [yep, you can put more than one apposite phrases in the same sentence] quite enough!

Find Out More...

• Avoiding Comma Confusion, by Moira Allen
  
  
• Connect with Conjunctions, by Victoria Grossack
  
  
• Handling Gerunds and Their Relatives, by Dawn Copeman
  
  
• Punctuating Dialogue, by Marg Gilks