Anisotropic 2D etch rate diagram:

Common etchants such as EDP, KOH, and TMAH have 2D etch rate diagrams 
in the (100) plane similar to this one. Note that each etchant has 
different 3D behavior. In addition, behavior changes with wafer 
orientation; a (110) wafer has six fold symmetry rather than 4-fold.
The two dimensional etch rate pattern is a projection of the three 
dimensional etch rate diagram onto the wafer plane. 

In the figure, the slowest planes are the (111) family, the 
fastest are usually the (311) family and the (110) and/or (100) families 
are intermediate.