Abstract: Internal connectors are configured to conformingly fit within each of the mating ends of adjacent duct elements. The internal connectors each comprise a sleeve having an outwardly extended wall disposed about a perimeter of the internal connector. The mating ends each have a stepped face configured to conformingly mate with the internal connector. To connect the duct elements together, the mating ends are mated to each side of the internal connector and the mating ends are glued to each other. Because the mating ends and the internal connectors conformingly mate with each other, the duct elements will be aligned with each other. The internal connectors are glued to a side wall of the duct element wherever branch take-offs are made.