We refer to these various values as $+\frac{1}{2}$, or $-\frac{1}{2}$, due to a peculiar property of the class of particles with half-integral spin, or fermions (to save one that mouthful): these particles are not invariant under 360 degrees of rotation around their axes, as one might expect from our experience of the macroscopic world, but rather require an additional 360 degrees of rotation, for a total of 720 degrees, in order to return to their original position.