A theory of the strong (nuclear) interactions among quarks, which are regarded as fundamental constituents of matter, structureless and indivisible at the current resolution of about 10⁻¹⁸ m. Quantum chromodynamics (QCD) explains why quarks combine in certain configurations to form the observed patterns of subnuclear particles, such as the proton and pi meson. According to this picture, the strong interactions among quarks are mediated by a set of force particles known as gluons. Quantum chromodynamics successfully accounts for many features of high-energy hard scattering of the strongly interacting particles. Interactions among gluons may lead to new structures that correspond to as-yet-undiscovered particles. The long-studied nuclear force that binds protons and neutrons together in atomic nuclei is regarded as a collective effect of the elementary interactions among constituents of the composite protons and neutrons.