Quark model of particle physics. Computer illustration showing the composite particles and how they are related in the quark model of particle physics. Quarks combine to form hadrons - composite particles categorised into two families: baryons - composed of three quarks, and mesons - composed of a quark-antiquark pair. Each hadron also contains gluons - particles that are responsible for joining the quarks and antiquarks to each other. The number of constituent quarks is not the only difference between baryons and mesons. A baryon hadron is known as a fermion and a meson hadron is known as a boson. Fermions are particles that have half-integer spin, whereas bosons have integer spin. Each form of hadron (baryon, or meson) also has a large number of