Avid Game of Thrones fans may remember Stannis Baratheon’s commitment to correcting the word less to fewer. The habit no doubt warmed the heart of every fussy grammarian watching the show—and maybe elicited a few eye rolls from other viewers.

Stannis was right: There is a so-called “correct” way to use each term. But like other grammar rules, this one wasn’t ever really set in stone, and it also comes with a few exceptions.

When to Use Fewer vs. Less

The rule itself is pretty simple. Use fewer to modify things you count the number of, and use less to modify things you measure the amount of. You ordered fewer pancakes than your sister, so you needed less syrup. There were fewer people in the diner today than there were yesterday, so it took less time to be seated. Another way to think of it is in terms of singular versus plural. Less usually modifies singular nouns, while fewer pairs with plural ones.

People usually err on the side of less. You probably wouldn’t accidentally say “fewer syrup” or “fewer time,” but you might say “less pancakes” or “less people.” Here’s an easy way to keep the terms straight: Put three before the noun in question. If it makes sense—three pancakes, three people—go with fewer. If it sounds weird—three syrup, three time—then stick with less.

Exceptions to the Rule: Less Than vs. Fewer Than

The exceptions to the rule typically come into play when you’re deciding between fewer than and less than. According to Merriam-Webster, less than is often used for these categories of countable things:

• Distance (e.g. less than 100 yards)
• Money (e.g. less than $100)
• Time (e.g. less than 24 hours)
• Weight (e.g. less than 100 pounds)
• Statistical enumerations (e.g. less than 10,000 people)

The common trend here is that while the modified noun itself is countable, the category isn’t. When you say “less than 100 pounds,” for example, you’re really saying “less weight than 100 pounds.” Weight is what’s being measured; pounds are just a replaceable unit of measurement being used to do it.

In these situations, it can help to think about fractions and mixed units of measurement. Saying “fewer than 24 hours” implies an exact number of hours—but you might actually be talking about 23 hours, 13 minutes, and 3 seconds. Again, what you’re measuring is time, not hours, so less than 24 hours is the better option.

One situation in which this rationalization doesn’t work quite as well is when there are people involved: They usually are counted one by one (averages and percentages excluded) and can’t easily be swapped out for some other unit of measurement. But in many cases, you can still come up with an uncountable noun that justifies the use of less than over fewer than. If you say a stadium can hold less than 10,000 people, you’re measuring an amount of space; if you say less than 10,000 people responded to a poll, you’re measuring an amount of engagement.

Another blanket exception to the fewer-versus-less rule involves the phrase one less. Even if you can count the thing, you still use one less. Nobody says “one fewer problem” or “one fewer lonely girl.”

The History of the Rule

The rule dates back to 1770, when an English writer named Robert Baker covered it in his book Reflections on the English Language:

“[Less] is most commonly used in speaking of a Number; where I should think Fewer would do better. No fewer than a Hundred appears to me not only more elegant than No less than a hundred, but more strictly proper.”

The 1994 edition of Merriam-Webster’s Dictionary of English Usage points out how subjective this is. “Baker’s remarks about fewer express clearly and modestly—“I should think,” “appears to me”—his own taste and preference,” it reads.

It’s not clear how Baker’s opinion became a governing grammar law that we still abide by today. But we do know that people have been using less rather than fewer to modify countable items for at least 1100 years—so don’t let the “10 Items or Less” check-out sign at the grocery store upset you too much.

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This article was originally published on www.mentalfloss.com as Fewer vs. Less: When to Use Each.