Prismanes have been shown to exhibit negative Poisson's ratio (auxetic behavior) for their internal rings and normal Poisson's behavior for the terminal rings. Various , , , and  biprismanes were studied by using quantum mechanical ab initio calculations. The unusual mechanical behavior of prismanes upon stretching was traced to the relaxation of geometrical distortion existing in the ground state. The increase of the Poissonic effect on the ring size correlates well with the Thorpe-Ingold effect. In biprismanes where the CH units in the external rings are replaced by nitrogen, the effect is accentuated. Substitution of the hydrogens by fluorine atoms markedly reduces the effect, this being an interesting example of substituent effect on mechanical properties. This effect is attenuated in longer prismanes.