These cast aluminum brackets are used to connect pieces of 20mm aluminum extrusion at a 90 degree angle. The gussets guarantee a solid, rigid connection. We recommend M5 8mm button head or low profile screws and any M5 t slot nuts that are compatible with 20mm extrusion (neither are included). Build CNC routers, 3D printers, CNC laser cutters, CNC plasma cutters, robotics projects, 3D carvers, machine guards, workstations, etc.