Is there a three-dimensional equivalent of the Pythagorean theorem? It all depends, of course, what one means by equivalent. There are in fact several distinct results which can with some justice be regarded as threedimensional equivalents of the theorem.

One interpretation of the Pythagorean theorem is as a statement about any rectangle, expressing the square of its diagonal as the sum of the squares of its height and length. This readily extends to the case of a rectangular box (cuboid), for which the square of its long diagonal always equals the sum of the squares of its height, length and depth.