Quarks are never observed in isolation: they occur only as a system of three quarks (called a baryon), or as a quark-antiquark pair (called a meson). (An antiquark is a form of antimatter, described b...Quarks are never observed in isolation: they occur only as a system of three quarks (called a baryon), or as a quark-antiquark pair (called a meson). (An antiquark is a form of antimatter, described below.) Examples of baryons are the proton (which consists of two "up" quarks and one "down" quark) and the neutron (which consists of two "down" quarks and one "up" quark).
