  Let $\{P_1, P_2, P_3, P_4\}$ be a quadruplet of points in $S^3$ . We define a
``dual'' quadruplet of it in a conformal geometric way. We show that the dual
of a dual quadruplet coincides with the original one. We also show that the
cross ratio of the dual quadruplet is equal to the complex conjugate of that of
the original one.
