A down quark is a fundamental particle that is one of the building blocks of matter. It is a type of quark, which are elementary particles that combine to form protons and neutrons, the components of atomic nuclei. The down quark has a negative electric charge of -1/3 and is one of the six types of quarks, the others being up, charm, strange, top, and bottom. Down quarks are found in pairs with up quarks in protons and neutrons. For example, a proton consists of two up quarks and one down quark, while a neutron contains one up quark and two down quarks. These combinations are held together by the strong force, mediated by particles called gluons.