The RAM Rail Base designed to sit at right angles to the mounting rail. The Stainless Steel U-Bolt and nuts included will fit a circular rail with diameter from 3/4" to 1.0". To mount the rail base, place the top of the base on the rail and then slide the u-bolt around the rail, into the holes in the base top. Now tighten the screws onto the u-bolt until the base is clamped firmly. If you desire, you can now slot the black plactice covers over the exposed ends of the u-bolt. The base is completely rust proof.