What is Pythagoras theory used for?

He came up with the Pythagoras theorem

In a right angled triangle the square on the hypotenuse is equal to the squares on the other two sides.

Builders make use of the theorem to make corners square. If you have piece of rope or chain 12 metres long and make a triangle on the ground where one side is 3 metres , another 4 and the longest 5 then a right angle will be formed between the 3 and 4 metre lengths.