The striking success of the parton model in describing deep inelastic–nucleon and neutrino–nucleon scattering provided strong circumstantial evidence for the Feynman–Bjorken picture and for its complete elaboration as quantum chromodynamics (QCD). QCD describes all strong interactions as resulting from the interactions of spin-1/2 quarks and spin-1 gluons. The fundamental coupling is of the gluon to the quarks, in a fashion analogous to the coupling of a photon to electrons. In addition, the gluons couple directly to each other. SU(3) plays a central role. Just as in the Gell-Mann–Zweig model of hadrons, there are three basic constituents. The u quark, for example, comes in three versions, say, red, blue, and green. Similarly, every other kind of quark comes in these three versions or “colors.” Often it is convenient to refer to u, d, s, and c as “flavors” of quarks, to contrast with the three colors in which every flavor comes. While the SU(3) of flavor is an approximate symmetry, the SU(3) of color is an exact symmetry, thus the three colors of the u quark are exactly degenerate in mass, while the u, d, and s quarks are not degenerate.

The rules of SU(3) state that if we combine a 3 (a quark) with a 3* (an antiquark), we get 1 + 8, a singlet and an octet. In terms of mesons, this explains that combining the three quark flavors (u, d, s) with the three antiquark flavors yields an SU(3) singlet (η′) and an octet (the pseudoscalar octet of π, K, η).