If you were going to make a list of situations in which people get confused about the use of apostrophes—and I’m not suggesting you should make such a list, but if you were—I think “its” versus “it’s” would be near if not at the top of the list.

Here’s the rule: If the “its” is possessive, there’s no apostrophe. If the “it’s” is a contraction for “it is” or “it has,” then there is an apostrophe.

Perhaps you are now thinking, “So why is that so confusing?” Well, the apostrophe is used both for possession and contractions in English—and that can make things confusing when it comes to the pronoun “it.”

The apostrophe is used for possessives with all nouns, but not pronouns. So we would say, “Anne’s book” with an apostrophe, but we would say, “The book is hers,” no apostrophe. “It,” because it is a pronoun, takes the “s” with no apostrophe when it’s possessive.

Meanwhile, we also use the apostrophe for contractions, as in words like “can’t” (for “cannot”), “don’t” (for “do not”), and “she’s” (for “she is”). Following this pattern, “it’s” with the apostrophe stands for the contracted form of “it is” or “it has” (e.g., “it’s been done”).

People sometimes feel dismay that so many writers have trouble distinguishing between “its” and “it’s.” I sense that behind this dismay is a feeling that there was some moment in the history of English when everybody knew how to use an apostrophe; we have since passed that moment and now nobody knows how to use an apostrophe consistently or correctly.

I’m here to tell you that I don’t think there ever was a moment in the history of English when everybody knew how to use an apostrophe—or at least when everybody was using apostrophes in exactly the same way.

The apostrophe has been unstable for its entire history. It’s been used for contraction. It’s been used for possession. It’s even been used for plurals at different points in English.

Right now, you can’t find full consensus in style guides about how to handle things like the plural “1930s” versus “1930’s” or the possessive “James'” versus “James’s.”

There is a city in England that has banned apostrophes on all of its street signs. McDonald’s loses its apostrophe on the web in its URL, and Pike’s Peak lost its apostrophe in 1891 when the U.S. Board on Geographic Names removed possessive apostrophes from the official names of most geographic places (e.g., Harpers Ferry).

Some thought texting might kill the apostrophe, but smartphones now put apostrophes in, both where they belong and where they don’t. The fact that we continue to see some inconsistency in apostrophe use and some changes in official or standard conventions is not surprising to me as a historian of the language. The apostrophe’s unstable history continues all around us.

This video originally appeared in LSA Today, where you can find more videos, including an archive of Anne Curzan’s discussions of language.