<p>Porous silicon birefringence originating from preferred pore etching directions along the crystallographic axes has been reported in a number of scientific publications along the last three decades. This birefringence originates from the fact that the preferred pore etching directions are those along the main crystallographic axes, namely [100], [010] and [001]. The preferred surface orientation for the studies on birefringent porous Si is [110], where the main optical axis is horizontal and, for a light beam propagating along the direction perpendicular to the surface, there are two distinct refractive indexes, so that this configuration has been considered easier for actual devices fabrication and is, by large, the most studied as far as birefringence is concerned. In this work, centered on (100)-oriented porous silicon, we show that the nature of porous silicon birefringence depends on the porous structure configuration and is therefore also dependent on the orientation of the surface of the bulk Si used. Moreover, we demonstrate that such structural dependence is so large that even apparently similar configurations can lead to positive or negative birefringence. Possible reasons for these remarkable differences are discussed along the experimental results.</p>