In the figure 1 following, the knot 9Aj has a triangular section Prr cutting away on each side of it a (3 + r)-gonal mesh, and 9Ar2 has such a section Rpp, through one crossing only. Make in these knots creases at rr that approach to meet at R in 2, and creases pp that meet at P in 2. In 3, 2 is prepared for a rotation through two right angles about the fixed axis PE, through the crossing P and the crease-kiss R, or, as a more learned man would say, through the decussation P and the plicatorial osculation R–taking 2 for 9Aj: exchange here P and R if 2 is 9Ar2.