U, C, T quarks carry partial electric charges of +2/3; D, S, B quarks carry partial charges of -1/3. For antiquarks (not shown), the sign of the electric charge is reversed. The U, D ("up", "down") quark "family" is the lightest, exclusively comprising ordinary matter, including the hottest stars (the quark flavor composition of the neutron is DDU, the proton DUU). The C, S ("charm", "strange") family is intermediate in mass, and the T, B ("top", "bottom") quark family is the heaviest. Heavy quark combinations (more commonly found as mesons, or quark-antiquark pairs) exist only momentarily in our particle accelerators, in collisions between matter and "cosmic rays", during the "Big Bang", in supernovas, the environs of black holes, or as products of other extreme astrophysical phenomena.