Used to support the 6x4 and 4x4 systems attaching to any 3/8" (9.5mm) x 1 1/2" (38.1mm) or 3/8" (9.5mm) x 2" (50.8mm) ladder rack rail. Adjustable to nine incremental positions from 4" (101mm) to 12" (305mm) above or below the ladder rack. No threaded rod required. The pre-assembled slide clamps enable fast assembly to channel. Use 7/16" nut driver on slide clamps. .. see full description