Board-on-board fencing is a full privacy fence style where boards directly touch and overlap with no gaps. You will only be able to see the backer rails on one side of the fence.  Shadowbox fencing is semi-private, with boards alternating from one side to the other of the rails. This design means the vertical boards don’t touch. There will be slight gaps between the boards that you can look through at an angle. Shadowbox fences look the same from either side of the fence.