Oh! It uses "the square of the hypotenuse is equal to the sums of the squares of the other two sides" rule. I do know that one. I just wasn't visualizing what you were describing because I didn't realize you you meant to measure from the corner and then measure the hypotenuse. (And I didn't even have to use spellcheck for that! Woohoo)
I always remember that rule because I know a joke about a mathematician going to an Indian reservation and the punchline is "The squaw of the hippopotamus is equal to the sons of the squaws of the other two hides".
Just in case anyone wondered...
The Planes Indians practiced polygamy, and one chief had three squaws.
The first squaw lived in a teepee of elk hide, the second in a teepee
made of buffalo hide, and the youngest in a teepee of hippopotamus hide.
Then he slept with each wife on the eve of his great hunting trip.
He was gone nine moons and when he returned, he went into the elk hide
teepee and found that his wife had borne him a son. Likewise, in the
buffalo hide teepee, that squaw, too, had borne him a son. So, imagine
his surprise when he found twin baby boys in the hippopotamus hide
teepee.
This just proves that …
The squaw of the hippopotamus is equal to the sum of the squaws of the other two hides.