Up (u)charge +⅔; in protons and neutrons
Down (d)charge −⅓; in protons and neutrons
Strange (s)charge −⅓; heavier
Charm (c)charge +⅔; heavier still
Bottom (b)charge −⅓; very heavy
Top (t)charge +⅔; the heaviest known particle
Quarks are held together by the strong force, carried by gluons. They are never found alone.