Abstract: A sign box joining device for slidable engaging and interlocking with at least one extrusion member. The joining device having an elongate, flat, top wall having downwardly extending side walls at opposite sides of the top wall and inwardly extending bottom wall flanges at bottom ends of the downwardly extending side walls. The extrusion member includes an elongate, flat, bottom wall having upwardly extending side walls at opposite sides of the bottom wall, one upwardly extending side wall having an inwardly extending top wall and the other upwardly extending side wall having and outwardly extending top wall. The joining device slidably engages and interlocks the pair of extrusion members when a portion of the top wall is in contact with a portion of the inwardly extending top wall of both extrusion members.