Abstract: A jamb assembly is used to form a door frame system for installation within a wall having an opening for a door. The jamb assembly comprises a first elongated member having an L-shaped shoulder along one side of the member. A groove running parallel to the shoulder extends the length of the member. The jamb assembly has a second elongated member identical to the first member. The members are inverted relative to one another so that the L-shaped shoulders of the first and second members fit together to form an expandable base of the jamb assembly. A third elongated member has a tongue projecting on one side thereof that extends the length of the third member. The tongue is formed to fit into the groove of the first or the second elongated member to form a cap for the jamb assembly. The tongue extends beyond the ends of the first and second members when the jamb assembly serves as a head jamb assembly.