Pythagoras theorem is relates the sides of a triangle.

In a right angled triangle the sum of the squares of the sides that contain the right angle is equal to the square on the the third side.

Let ABC be a right angled triangle with right angle at B. Then AB , BC are the sides that contain right angle . Then Ac is the side opposite to the right angle. Then AC^2 = AB^2+BC^2.

It is also true that in any triangle ABC if sum of the squares of any two sides AB and BC is equal to the third side AC, then the triangle is a right angled triangle with right angle contained by AB and BC.

Pythagoras theorem could be used to verify whether a triangle is right angle. It could be used to create 90 degree measure of angle with ordinary scale (not even calibrated) .