In theoretical physics, the Chan–Paton factor is a multivalued index associated with the endpoints of an open string. An open string can be interpreted as a fluxtube connecting a quark and its antiparticle. The two Chan–Paton factors make the string transform as a tensor under a gauge group whose charges are carried by the endpoints of the strings.
After the Second Superstring Revolution in 1995, Chan–Paton factors are interpreted as labels that identify which (spacetime-filling) D-branes the stringy endpoints are attached to. The Chan–Paton factors have become a special case of a more general concept.