 For two light quarks, the up quark and the down quark, the QCD Lagrangian provides insight.The symmetry of the QCD Lagrangian, called "chiral symmetry" describes invariance with respect to a symmetry group formula_2.This symmetry group amounts to

The quark condensate induced by nonperturbative strong interactions spontaneously breaks the formula_4 down to the diagonal vector subgroup "SU(2)V", known as isospin.