Abstract: A retractable stop assembly rigidly attached to an underside of a cart for sheet material. The retractable stop assembly includes a base plate member adapted for rigid attachment to an underside of a cart adjacent an edge thereof. A mounting bracket unit is rotatably attached to the base plate member opposite the underside of the cart by a threaded fastener. The mounting bracket unit includes a cylindrical sleeve member rigidly secured thereto with the cylindrical sleeve member's central axis parallel with the base plate member. An L-shaped stop member has first and second leg sections. The first leg section is movably positioned within the cylindrical sleeve member, and the second leg section is movable to extend above the edge of the cart. Rotation of the threaded fastener member in a selected direction rigidly clamps the L-shaped stop member in a selected orientation within the cylindrical sleeve member.