Introduction to Quark Model
The fundamental representation of the isospin group SU(2) is the doublet representation. The proton and the neutron are described by a doublet. The fundamental representation of SU(3) is the triplet representation, but in nature there are no hadrons, which transform as a triplet—the hadrons are described only by singlets, octets and decuplets. This feature of the SU(3) symmetry was not understood until 1964.