One of Niels Bohr's main contributions to Physics was his assertion (backed up by experiment) that angular momentum is quantized - it can only occur in integer multiples of . Erwin Schr´'odinger showed why this was true for the wave functions of the hydrogen atom, but by that time Bohr's principle had been elevated to an empirical ``law'' of Physics that went well beyond the realm of atoms. Schr´'odinger also showed the peculiar nature of the quantization of : first, its magnitude obeys where can only have integer values from zero to , n being the principle quantum number for which in the case of hydrogen; second, its projection onto the z axis obeys where can take on only integer values from to . Note that Bohr's original prescription for angular momentum quantization (integer multiples of ) is actually applicable to the z component of - its projection onto the z quantization axis, which is chosen arbitrarily unless there is a magnetic field applied, in which case is always chosen along the field, .