Quark models, first proposed in 1964 independently by Murray Gell-Mann and George Zweig (who called quarks "aces"), describe the known hadrons as composed of valence quarks and/or antiquarks, tightly bound by the color force, which is mediated by gluons. (The interaction between quarks and gluons is described by the theory of quantum chromodynamics.) A "sea" of virtual quark-antiquark pairs is also present in each hadron.