When it comes to punctuation marks, the one that is probably victim to the most errors is the apostrophe. Because of its widespread use, it is much more common to find a mistake with an apostrophe than it is to find a mistake using a colon or semicolon. Chances are, all you have to do is head to your local grocery to find someone misusing and abusing the apostrophe. (Want to buy banana’s and check out the sale’s?)
Generally speaking, there are three scenarios where you would want to use an apostrophe: possession, omission, and pluralization. It is largely because these three situations appear so similar that many people find themselves over-correcting and placing apostrophes where they don’t belong… like in the example displayed above. You don’t need an apostrophe to pluralize the word “apostrophe.” It’s just “apostrophes.”
Apostrophes to Indicate Possession
Since I bought an Xbox 360, you could point to the system and say that it is Michael’s Xbox 360. You see how I added an apostrophe and the letter “s” after my name to indicate possession of the noun that follows? This the exact same mechanism that you would use to refer to John’s website, Stephen’s scooter, or Carl’s kitten.
Now, there is still an unresolved debate over how you would use this construction if the owner or possessor ends in the letter S. For example, if you wanted to talk about a bicycle that belonged to Chris, some people say that you should write it as “Chris’ bicycle,” whereas others remark that it should be “Chris’s bicycle.” I personally prefer the former, because it looks cleaner in print, but I tend to vocalize it like the latter.
By contrast, if the noun is plural, I almost always use the former method. If I wanted to refer to a lunch possessed by several dot com moguls, I would write it as “the dot com moguls’ lunch” rather than “the dot com moguls’s lunch”. Note that if I wrote it as “the dot com mogul’s lunch”, it would be the lunch possessed by a single dot com mogul.
Apostrophes for Missing Letters and Numbers
Apostrophes are also used when letters and/or numbers are omitted for whatever reason. The most common scenario would be in the use of contractions like should’ve, you’re, and it’s. These are shortened versions of should have (not should of), you are, and it is.
I should note that when most people refer to certain decades, they are actually placing the apostrophe in the wrong place. If you are referring to the years between 1990 and 1999, it would be called the ’90s and not the 90’s. This is because the “19” is being omitted when you use this construction, so the apostrophe should be placed where the “19” is missing. There is no need to put an apostrophe after “90” for pluralization, because the apostrophe is being used neither to show possession nor to indicate the omission of numbers/letters.
Discretionary Plurals with Apostrophes
And here is where it gets even more confusing. In general, you can use an apostrophe and an “s” when you are pluralizing a lower-case letter. A common expression is that you should mind your p’s and q’s. The apostrophes here are largely for ease of reading, because it would look odd to write it as minding your ps and qs.
When pluralizing a number or a capital letter, however, you do not need an apostrophe. As in the example I discussed earlier, it’s the 1990s and not the the 1990’s. That’s also why it would be technically correct to pluralize “iPhone 3G” as “iPhone 3Gs.” I know, it looks terrible. Some people have tried doing it as “iPhones 3G”, like how you could say brothers-in-law rather than brother-in-laws. The pluralization of iPhone 3G is still up to debate.
For an incredibly in-depth discussion on the apostrophe, its origins, its use, and its misuse, you can check out the Wikipedia page on the matter. I’m surprised how long that page has become.