 Fundamental particles of the same kind, for example electrons, are treated in quantum mechanics as if they are somehow identical or indistinguishable. This treatment is reflected in the symmetry properties of the wave function of a many-particle system under permutations of the coordinates of those particles. For electrons, and for other fermions, the wave function must change its sign when the coordinates of a pair of particles are exchanged; it must be antisymmetric. For photons and other bosons it must be symmetric, with no change at all.