attempting to explain the underlying reasons for these properties. Of course, we now know that these properties ultimately derive from the electron shell structure of the elements as dictated by quantum mechanics. Similarly, the utility of the quark model lies in its explanation of the large number of hadrons in terms of a small number quarks. The quark model also led to an important insight into the nature of hadrons. Consider for instance the Δ ++ baryon, which has spin-3 / 2. In the quark model, it is composed of three identical spin-1 / 2 up quarks that are not orbitally excited with respect to one another. This means that the spin, flavour and spatial wave functions are symmetric under particle interchange, meaning that the total wave function is also. Because the Δ ++ is a fermion, this violates the spin-statistics theorem. This observation led to the introduction of the colour quantum number for quarks [57], which in the case of the Δ ++ has an anti-symmetric wave function, thus ensuring that the spin-statistics theorem is upheld. The name colour was chosen in order to emphasize a key feature of hadrons, and is only used as an analogy. The number of colours can be inferred from experimental data, such as from the decay rate of the neutral pion [89]. An individual quark may have one of three colours: red, green or blue. Each of the three quarks within a baryon must have a unique colour, thus the combination of red, green and blue is considered to have no net colour charge. All baryons and mesons are colourless, or colour singlets. In QCD, the colour quantum number of quarks is understood as a kind of generalization of electric charge. Quantum Electrodynamics (QED) describes electromagnetic interactions between electrically charged particles that are mediated by electrically neutral photons. In QCD, quarks with colour charge interact via gluons, which also carry colour charge. This fact means that QCD is radically different from QED, and it is also responsible for many of the interesting features of QCD. Unlike QED, the fundamental degrees of freedom in QCD are not directly manifested in nature. Instead, quarks and gluons are realized in terms of hadrons. QCD suggests the possibility of a far richer hadronic spectrum than the quark model. Exotic hadrons are colour singlet hadrons that are neither baryons nor mesons (see, e.g. 1.2.3 Exotic Hadrons 6