Bill Tindall, E.Tn.
It is critical that the joining boards bottom out on the outside show edge, which is to say no internal part of the joint should stick up higher than the edge. Before assembly I always check with a square as shown in the picture. As I was sliding the square along the sockets of the pin board I noticed that the edge of the square was burnishing the end grain. The far middle of the closest socket has a burnish(shiny spot) indicating that it is high here, as well as the middle of the next socket and the 3rd is burnished at the outside edge as it needs to be. (see Picture) I realized that rubbing the square provided a means of determining if there were any high places in the joint by burnishing them. I have never seen this tip discussed so I thought I would mention it.