rule of three

American  

noun

Mathematics.

1. the method of finding the fourth term in a proportion when three terms are given.

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rule of three British  

noun

1. a mathematical rule asserting that the value of one unknown quantity in a proportion is found by multiplying the denominator of each ratio by the numerator of the other

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Etymology

Origin of rule of three

First recorded in 1585–95

Example Sentences

But you’ve probably heard of the “rule of three.”

From Salon

So, my wife and I vowed to implement the rule of three.

From Salon

It’s tentatively called “The Rule of Three,” and just like “We the Animals,” it tells the story of three brothers.

From Los Angeles Times

“Halfway through writing that book, I was coming up with even more, and I thought, it’s not going to fit in one book. So I was thinking two books, and if it’s two books it might as well be three. The rule of three kicked in. It seemed to make sense — it wasn’t daunting, because I found the material so enriching and compelling.”

From Seattle Times

“I call it the ‘rule of three’,” he said.

From Seattle Times