This rule was derived from Pythagorean's Theory for right (90°) triangles whereby the hypotenuse squared = the sum of each of the other two legs squares, i.e. C2=A2+B2. As Darren pointed out, this is a handy way to set up, or confirm, a square corner for many applications.
Thoughts entering one's mind need not exit one's mouth!
As I age my memory fades .... and that's a load off my mind!
"We Live In The Land Of The Free, Only Because Of The Brave"
“The problems we face today are there because the people who work for a living are outnumbered by those who vote for a living."
"Socialism is a philosophy of failure, the creed of ignorance, and the gospel of envy, its inherent virtue is the equal sharing of misery." Winston Churchill