And after he has reduced to a uniform type those triangles which are not right angles and are affected by greater inequality, he concludes the second book with the reduction of the rectangle to the square, with an equation between the self equal, or the square, and the internally unequal, or the rectangle; similarly, in the Pythagorean theorem, the hypotenuse that corresponds to the right angle, the self equal, constitutes one side of the equation, while the other side is made up by the self unequal, the two perpendicular sides.