If a point particle couples to the Maxwell field then that particle carries electric charge. Strings couple to the Kalb–Ramond field; therefore, strings carry a new kind of charge – string charge. For a stretched string, string charge can be visualized as a current flowing along the string. Strings can end on D-branes without violating conservation of string charge because the string endpoints carry electric charge and the resulting electric field lines on a D-brane carry string charge. Certain D-branes in superstring theory carry electric charge for Ramond–Ramond fields. If a charged brane is fully wrapped on a compact space, it appears to a lower-dimensional observer as a point particle carrying the electric charge of a Maxwell field that arises from dimensional reduction. Configurations of intersecting D6-branes in type IIA superstring theory define string models of particle physics that reproduce the full spectrum of gauge bosons and chiral fermions of the Standard Model.