This Trefoil Knot has a square cross section. Its shape is that of a piece of rope which is tied in an overhand knot and then the loose ends of the rope are joined. It is a favourite motif in stone carvings, and decorative designs of every kind. To get the square cross section with a continuous edge, I made three wooden rings, separated them into six semicircles and joined them together again (the offset angles of the square cross sections had to be calculated carefully)