paragraph_text: Quarks were soon accepted to be the underlying elementary objects in the study of the structure of hadrons. In 1972 he introduced with Harald Fritzsch the quantum number 'color' and later, in a joint paper with Heinrich Leutwyler, the full theory of quantum chromodynamics (QCD) was released. The quark model is part of QCD, and has been robust enough to survive the discovery of other flavours of quarks.