A right-handed Cartesian coordinate system, illustrating the x (right-left), y (forward-backward) and z (up-down) axes relative to a human being. The most common relative directions are left, right, forward, backward, up, and down. No absolute direction corresponds to any of the relative directions. This is a consequence of the translational invariance of the laws of physics: nature, loosely speaking, behaves the same no matter what direction one moves. As demonstrated by the Michelson-Morley null result, there is no absolute inertial frame of reference. There are definite relationships between the relative directions, however. Left and right, forward and backward, and up and down are three complementary pairs orthogonal to each other.