Quark-line flavour rearrangement

A quark-line diagram can show how existing valence quarks are regrouped into outgoing hadrons while gluons exchange momentum and colour. Alternatively, a quark-antiquark pair can annihilate and create a different flavour pair with zero net flavour. Such drawings enforce global charge, baryon number and flavour constraints; they are not controlled perturbative calculations of an exclusive low-energy hadronic amplitude because binding is nonperturbative.