Using the definition of Street, we see that $\lambda : GV \To F$ is a pointwise right extension if and only if $\lambda^\op : F^\op \To G^\op V^\op$ is a pointwise left extension, hence applying the dualisation rule Theorem 1·l [0022] and Theorem 1·j [001Z] we obtain that a square $\vf : US \To VT$ in $\Cat$ is exact if and only if, for every pointwise right extension the composite triangle remains a pointwise right extension.