The shape they choose for the frame is a Möbius strip, which has the primary defining characteristic of having only one side and one edge. If you pick a row of keys to follow, after one loop around, you’ll be on the underside of the frame. Continue to go around another full turn and you’ll be right where you started. A similar math curiosity would be the klein bottle, which has a single surface.

Adam Savage on Tested nerding out about Möbius strips and klein bottles.