With hindsight, it is possible to pinpoint Mendeleev’s periodic table of the atomic elements in 1869 as the first hint of a more fundamental layer of matter common to all atoms, and responsible for giving them their properties. Half a century later, the discovery that atoms consisted of electrons encircling a nucleus confirmed this. By the middle of the 20th century, the structure of the nucleus was in its turn being revealed. The observation of a recurring pattern among the 30 or so hadrons known in the early 1960s was an analogous pointer to the possibility of a more fundamental variety of matter — quarks — out of which these hadrons, including the neutron, and proton, and ultimately the nucleus, are formed.