The up quark or u quark (symbol: u) is the lightest of all quarks, a type of elementary particle, and a major constituent of matter. It, along with the down quark, forms the neutrons (one up quark, two down quarks) and protons (two up quarks, one down quark) of atomic nuclei. It is part of the first generation of matter, has an electric charge of + "e" and a bare mass of . Like all quarks, the up quark is an elementary fermion with spin spin-1/2, and experiences all four fundamental interactions: gravitation, electromagnetism, weak interactions, and strong interactions. The antiparticle of the up quark is the up antiquark (sometimes called "antiup quark" or simply "antiup"), which differs from it only in that some of its properties, such as charge have equal magnitude but opposite sign.