Rules tell us what to do, and require no thought. Stop at a red light, or risk getting a ticket. Fill in only the circle in the test booklet, or risk being marked incorrect. Use “between” only for two things, and “among” for more than two, or risk being called a grammatical dullard.

Unfortunately, grammatical “rules” usually aren’t so black and white. The “rule” about when to use “between” and when to use “among” is pretty simple: Use “between” when thinking about one thing’s relationship with one other thing (even if there are several other things in the mix), and “among” when you’re thinking about the relationships of several things collectively.

So before you can decide which to use, you have to know about what you’re talking.

But it’s easier to remember a “rule” than figure out context. And that results in phrases like this: “[I]t is possible to earn a degree from an American university without setting foot in the United States, or by flitting among campuses.” The relationship is between one person and one campus at a time, not between one campus and another, so there are only two parties here.

The Associated Press Stylebook says, “The maxim that between introduces two items and among introduces more than two covers most questions about how to use these words: The funds were divided among Ford, Carter and McCarthy. However, between is the correct word when expressing the relationships of three or more items considered one pair at a time: Negotiations on a debate format are under way between the network and the Ford, Carter and McCarthy committees.”

Many people, though, just read that first part: “between introduces two items and among introduces more than two.”

This, from The New York Times Manual of Style and Usage, is a bit clearer. “In general, between applies to two things, and among to more than two. But between is correct in reference to more than two when the items are related individually as well as severally: Trade between the United States, Canada and Mexico has grown under Nafta. Each country trades with each of the others, rather than with all simultaneously. When more than two things are related in a purely collective and vague way, use among.”

If you’re still confused, you’re in good company. Many grammar texts, in an attempt to keep things simple for schoolchildren, oversimplify “between” and “among,” giving misleading advice. And for at least a couple of hundred years, some grammarians have insisted that “between” can be used only when two items are involved. The instant a third comes into play, they say, “among” is required, regardless of the relationships involved. But they’re overruled by common usage, and common sense.

Instead, maybe this, from Merriam-Webster’s Dictionary of English Usage, will help: “We suggest that in choosing between among and between you are going to be better off following your own instincts rather than trying to follow someone else’s theory of what is correct.”

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Merrill Perlman managed copy desks across the newsroom at The New York Times, where she worked for 25 years. Follow her on Twitter at @meperl.