A type of elementary particle which is the major constituent of matter. Quarks are never found on their own, only in groups of three within composite particles called hadrons (such as protons and neutrons). There are six different types (or “flavours”) of quarks - up, down, top, bottom, charm and strange - and each flavour comes in three “colours” - red, green or blue (although they have no colour in the normal sense, being much smaller than the wavelength of visible light). Quarks are the only particles in the standard model of particle physics to experience all four fundamental forces, and they have the properties of electric charge, colour charge, spin and mass.