Abstract: A seat assembly includes a molded plastic shell and an embedded frame. The seat assembly can include a seat bottom and a seat back which are connected by a curved backstop. The curve of the backstop is dictated by the embedded frame. The seat assembly can be part of a modular chair system, in which the seat assembly can be attached to a chair base, without or without optional arms. The seat assembly can be transferred between a low density stacking chair base and a high density stacking chair base without modification of the seat assembly. The arms can be attached to the chair base without modification of the chair base. A method for attaching a seat assembly to a chair base is also provided.