I’ve never had any trouble remembering the difference between “it’s” and “its” — but it’s one of the most common mistakes I see when proofreading other people’s stuff. And when I was a kid, no one could provide a satisfactory explanation for why “its” doesn’t get an apostrophe when it’s possessive. Of course, “it’s” gets an apostrophe when it’s a contraction of “it is,” but why wouldn’t “its” get an apostrophe when it’s possessive? After all, if I wanted to indicate that the book I finished reading last week belongs to my dad, I’d write that it’s “my dad’s book.” The possessive form of “dad” gets an apostrophe, so why doesn’t the possessive form of “it”?

It seemed to be an anomaly — a rule that existed for no good reason other than to make English that much more difficult to learn. Even in high school, an English teacher I confronted with this question could only restate that the no-apostrophe rule for possessive “its” existed, but not why it seemed to defy the rule that governed other possessive forms.

It wasn’t until midway through college, eight or so years ago, that I stumbled on the answer. It’s a pretty obvious answer in retrospect, but I finally felt vindicated — “its” as a possessive form without an apostrophe did follow a rule! It’s a rule that a few other possessives follow too: possessive pronouns never have apostrophes. Check ’em out:

its his hers mine yours ours theirs whose

The possessive “its” is probably mistakenly rendered as “it’s” so often because it’s the only possessive pronoun with a valid apostrophe’d doppelgänger — there’s no “their’s” in the English language (although it’s true there’s a “there’s,” it’s a contraction that uses a homophone of “their,” so it’s not a double in the same sense as “its” and “it’s”). Perhaps if English teachers made a point of pointing out to their classes this entire class of possessive pronouns, more adults would understand that it’s “its” for a reason.