live, where the up and down quarks have a tiny but nonzero mass, but it is a very good approximation, to the extent that isospin is conserved. Below a certain temperature which is the symmetry restoration temperature, the phase is ordered. The chiral condensate forms, and pions of small mass are produced. The SU(Nf) charges, Isospin and Hypercharge and SU(3), make sense. Above the QCD temperature lies a disordered phase where SU(Nf)×SU(Nf) and color SU(3) charges make sense. It is an open question whether the deconfinement temperature of QCD is also the temperature at which the chiral condensate melts. Notes References . . . Quantum field theory