Crystal planes:

Three of the most common planes in anisotropic etching are shown 
on the left. Each plane is defined in terms of its Miller indices 
which are the inverses of the axes intercepts. The (100), (110), and 
(111) planes intersect 1, 2, and 3 axes respectively.

The views perpendicular to these planes are shown on the right.
The planes have 4, 6, and 3 fold symmetries respectively. The etch rate 
diagram for each wafer orientation tends to have the same symmetry.
In addition the (111) planes are the densest and the denser planes tend to 
etch the slowest.