Historical proof of Pythagoras' theorem. Discovered at around 400BC in both China (where it is known as hsuan-thu) and Greece, this theorem states that the square of the hypotenuse on a right-angled triangle is equal to the sum of the squares of the other sides. The proof depicted demonstrates that the area of two squares, with sides determined by the shorter edges of the triangle, is the same as the area of a square with sides the same length as the triangle's hypotenuse. This theorem is one of the most important in geometry. During the Dark Ages a lot of classical knowledge was preserved in the Arabic world, which may explain the Arabic script.