A doubt that seems to bedevil the confidence of many writers is where, if anywhere, to put the apostrophe. Is it my dog’s collar or my dogs’ collar? It’s or its? The problem lies in the many duties we ask the apostrophe to perform, and sorting out two of the more common may begin to exorcise the trouble.
Very commonly, the apostrophe is there to show the possessor of something. If I complain that I cannot find the dog’s collar, I mean to tell you that I cannot find the collar that belongs to the dog. It may be true that I’m the one who put down the cash for it, but I bought it for the dog, and now he is the one to whom it belongs. I stress this point because the idea of possession is actually a little trickier than it may at first appear.
Possession always involves at least two entities: the possessor and the possessed. In English, the apostrophe points to the possessor (not all languages express possession that way), and remembering this can clear up one apostrophic problem quickly. So if the reality is that I have one dog and I cannot seem to find the collar that belongs to that one dog, then the dog’s collar is correct: the apostrophe is pointing back, so to speak, to the possessor. If, though, there had been a sale on collars one day and I had bought three of them for my one dog—and I cannot seem to find any of them, then my dog’s collars would be correct: the apostrophe still points to the possessor, my singular dog.
But the plot thickens. If I have a pack of dogs and I cannot find their collars, I would have to change the position of the apostrophe to point to the many possessors: the dogs’ collars, which means the many collars that belong to the many members of the unruly pack. And the only time it would be accurate to write my dogs’ collar would be to have only one collar for more than one dog (which, although one can never say never, would preclude my walking them all at the same time). To show possession, then, the apostrophe points to the possessor—most of the time.
But not always. Take the ruinously common confusion between its and it’s. Given all that we have just said about the apostrophe showing the possessor, one could reasonably infer that it would be correct to talk about my company’s family leave policy as it’s policy. But the word it is a personal pronoun (some others are I, you, he, she, they), and, as life would have it, English personal pronouns never use an apostrophe to indicate possession. So its policy it is.
It remains, then, to explain it’s. The apostrophe does more in English than simply indicate possession. It is often (very often) used to construct a contraction, the shortening of a word, phrase, or clause by omitting a letter, sound, or syllable. We see this use in words like can’t and don’t and haven’t. Thus, it’s is the contraction for it is, and it’s perfectly correct in It’s great to be on family leave.
Possession and contractions are two uses of the apostrophe. The mark is also used sometimes to form the plural of abbreviations (M.D.’s), and then there’s the matter of the possessive with the gerund. But that’s a longer story whose tale (or is it who’s tale?) will be told another time.