When I was making my shelf, I left the two end pieces for last so I could fit them around the legs. I also didn't put a supporting cleat on the side rails until I had made the end pieces. With the end piece in place, without the cleat underneath, I could see light coming through the gap between the side rail and the end piece. It looked very wrong. Dark shadow lines everywhere with tongue/groove or a cleat underneath, and then light sneaking through that one gap. The light is gone now since I added a side rail cleat (necessary because that is the only support that the end piece receive)