This is a simple block attached to the Mortise Rail 90 degrees to its face. It can be attached in any of the vertical tracks at any height so long as its fasteners engage the track. There are slots cut into the edge to hold a clamp in relative position while stock is being adjusted on the clamp. The block isn't all that big but it is big enough to take advantage of the Y-stroke of the Rat. This clamp could be used to make tenons, box joints, and many other things.