"A property called confinement [dictates that] the strength of the interaction becomes smaller the closer the particles are, rather than the further they are.  Accordingly, one can never see an isolated quark or gluon, only combinations of them.  Groups of three [...] are known as baryons, amongst which protons and neutrons are the most prominent examples. —Thank you, Andre Liddle & Jon Loveday, for The Oxford Companion to Cosmology [Online Version], in the "standard model for particle physics" entry, 2012.