The electron, proton, and neutron, as well as the mu meson, neutrino, Λ, Σ, and Ξ particles, all have an internal angular momentum, like a ball spinning on its own axis, with the property that if one measures the component of this angular momentum along any direction one finds either the value +ħ/2 or – ħ/2. Such an internal angular momentum, corresponding to a total angular momentum quantum number l equal to 1/2, is called spin ½.