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

There was a time when America’s greatest writers, wits and humorists were known by their last names: Parker, Benchley, Thurber and, breaking the comedy rule of three, Perelman.

From Washington Post • Aug. 24, 2021

By the rule of three, however, that means it's likely to be offered one more time.

From Salon • Apr. 10, 2019

The rule of three as the most stable structure can be subverted to afford you double the scapegoats.

From Slate • Apr. 2, 2019

The Nobel rule of three would mean someone was left out, unless the prize was somehow split between medicine and chemistry: unlikely but not impossible.

From The Guardian • Oct. 2, 2017

Each house consisted, as a rule, of three or four small sheds, facing inwards, and forming a tiny courtyard.

From The Great White Queen A Tale of Treasure and Treason by Le Queux, William