A quark is a type of elementary particle that underlies protons and neutrons: each is comprised of three quarks. This structure is key to the standard model, which explains much of the observed interaction at the sub-atomic level (particle physics). Quarks come in six types (termed flavors) called up, down, strange, charm, top, and bottom, and for each of these flavors, there is an antimatter equivalent, i.e., a corresponding antiquark. Quarks' spin is 1/2 and their electric charge is either +2/3 or -1/3 that of an electron. A proton's three quarks are two up and one down, and a neutron's are two down and one up. Particles made of two quarks are termed mesons. Quarks are subject to the strong force, whose particle representation is the gluon (just as electromagnetic radiation's particle representation is the photon) and it is this strong force that holds together the quarks that comprise protons and neutrons. Under some extreme circumstances, it is assumed protons and neutrons come apart, resulting in matter not made up of nuclei but of quarks, termed quark matter. Possible examples of this are the extreme high temperature of the very early universe, and the extreme density and temperature of the interior of massive neutron stars.