Do you mean where it got it's name from? The Pythagorean theorem got its name from the Greek mathematician Pythagoras. If not then maybe you meant how it works? It is used to solve right triangles. It states, "that the sum of the squares of the lengths of the two other sides of any right triangle will equal the square of the length of the hypotenuse" or a^2 + b^2 = c^2 (Written as "a" squared plus "b" squared equals "c" squared). Integers that meet these conditions are called Pythagorean triples. If you meant something other than this then you need to specify what you wanted to be answered in the question for the history of it is to generalized to find something specific.