These sliding units mount on fixture plates to allow for both horizontal and vertical adjustments of clamps, supports, rests, etc. The center block slides on the bottom block and the top block slides on the center block. Tightening the locking screws fastens the top or center block after positioning. The center and bottom blocks are graduated for reading and recording travel for easier set up. Mounts using socket head cap screws. Precision ground steel, black oxide finish.