X-Rail Screw Plates are a longer version of the X-Rail Nuts.  When you want to attach an Actobotics component using the 0.770" or 1.50" (measuring 1.061" to align with the side) pattern, X-Rail Screw Plates provide more surface contact with the slot of the X-Rail when compared to the X-Rail Nuts.  These screw plates are able to move to any location along the X-Rail and lock into place when a part is fastened using 6-32 screws.