It seems like I can't even make it through a whole day without seeing a poor little apostrophe being abused. Some people seem to think they're just a fancy accessory for any word that ends with the letter S, and that's just not right. This quick little workshop is meant to help those poor, confused souls understand how to properly use their apostrophes so that this horror can end. Plural Words The short explanation for how apostrophes and pluralized words work together is this: they don't. The slightly longer explanation is that apostrophes simply aren't used to show that there is more than one of something; that is what the letter S added to the end of the word is for. For words that already end in S, you generally add 'es' to it in order to show plurality, such as in the words 'classes' and 'masses.' If you want to tell people that you have more than one cat, you wouldn't write “I have five cat's,” you would write “I have five cats.” Or, well, you'd write it the second way if you cared about being grammatically accurate, at least. I'll assume that you do, because you're bothering to read this post. Possessive Words This is one of the two major proper uses of apostrophes. Throwing 's on the end of a word will, in most circumstances, indicate that some other part of the sentence will refer to something that belongs to that modified word. It doesn't necessarily have to be a physical possession that you refer to here, because you would use the possessive 's addition to refer to something like Einstein's Theory of Relativity or Nazi Germany's crimes against humanity, but it will always be something that is clearly owned or possessed by that person or thing or whatever that you added the 's to. If you want to say that something belongs to a particular entity and don't want to write something like “the hat belonging to Jim,” then you should use that possessive form and call it “Jim's hat.” Contractions This is the other major proper use of apostrophes. In this usage an apostrophe is put in the place of omitted letters in a word in order to shorten it or alter it for other purposes. For example, the apostrophe in “don't” is replacing an O from “do not” and the apostrophe in “can't” is replacing “no” from “cannot.” This kind of apostrophe usage can also be found in more colloquial terms like “fuck 'em” and “'ello mates,” generally written in order to accurately record (if the words are quoted from someone speaking them) or imitate (if writing a character who talks like that) a particular type or speech that isn't quite grammatically accurate. Important Exceptions and Oddities Like any set of rules for the English language, apostrophe usage has some exceptions and weird things that don't follow the rules but are still considered grammatically accurate. Luckily, the only major area of weird but accurate uses for apostrophes comes from pronouns, because they're words that have both common possessive forms and common contraction forms. In these instances the version with the apostrophe will be the contraction and an alternative form will take over possessive duties. For example: It's = contraction of it is or it has. EX: It's an ugly coat. Its = possessive, thing belonging to it. EX: Its sleeves are far too small. Who's = contraction of who is or who has. EX: Who's farting? Whose = contraction of possessive, thing belonging to who. EX: Whose stench is this? You're = you are. EX: You're a big stupid face. Your = possessive, thing belonging to you. EX: Your face looks like an ass. They're = contraction of they are. EX: They're actually gang members, you know. Their = possessive, thing belonging to them. EX: Their mothers are so proud. Pretty much all English pronouns follow the same rules as above. 'His' is possessive while “he's” means he is, “hers” is possessive while “she's” means she is, and so on. Other Uses In a lot of fonts you'll find that single quotation marks, such as around 'this' word, will be identical to apostrophes. This common appearance is fine unless they're touching an apostrophe and might cause some confusion, in which case you ought to change something to remove the confusion, perhaps use full quotation marks instead or alter the sentence to move things around. It's considered grammatically okay to use an apostrophe when referring to single letters to avoid possible confusion. For instance, saying “Betty got all A's and B's this term” might potentially cause some confusion if written as “Betty got all As and Bs this term.” This is not mandatory though, so if you can refer to single letters without an apostrophe without causing confusion (such as how in the intro to this post I used the phrase “the letter S” to avoid the apostrophe thing) then you're fine without it. Grammar rules books used to (until fairly recently) say you should always use an apostrophe when referring to things that are all capital letters or numbers, such as “ATV's” or “1990's.” This has sort of gone out of style though, so “ATVs” and “1990s” are fine as is, but it's not technically wrong to use the apostrophe in those places. This has become one of those stylistic choices, so do whatever works for you. Also, quick note, contractions rules apply to years and such as well, so write the decade as the '90s (or '90's) if you want to drop the 19 part. TL;DR Conclusion If you aren't referring to a possession and you aren't using a contraction, then you almost certainly don't need to put an apostrophe in whatever word you're considering mangling. Simple enough, yeah?