This is a template that can be used interactively, or by printing and cutting out, to demonstrate Pythagoras' theorem. The four congruent triangles can be arranged in the square in many ways, but two in particular are important:
1) Arrange the triangles around the outside so the remaining space in the middle is a square with side length the hypotenuse.
2) Arrange the triangles so the remaining space is split into two squares, one with side length the shortest side, and one with side length the middle side.
The areas of the two arrangements are equal, so the square on/of the largest side is equal to the sum of the squares on/of the other two sides.
