The 3/16" thick base boards are beveled at 45 degrees to fit together, so all pieces cover the length or width. (more likely, a single board was cut, routed at the three corner/fold points, and folded into a rectangle.)
The base is a rectangle 13 3/4" x 7 1/2"
The front of the base is 2"
The back of the base is 3 1/4"