"Relativistic quantum theory predicts that particles having must come in pairs with opposite charges but identical masses (and identical lifetimes if unstable). One member of the pair is called the particle, the other the antiparticle. Which is called by which name is a matter of history and convenience. It turns out that there are other kinds of "charge" in addition to electric charge; for example, so-called . The necessity of particle-antiparticle paris obtains for charges of any kind. Thus, not only is there an antiproton to the proton, there is an antineutrino to the neutron. The neutron is electrically neutral but it has baryon number charge."