Mask shapes and etch anisotropy of Si in a particular etchant determine the final forms of the etched structures. This paper considers the etching of <100> silicon in tetra-methyl ammonium hydroxide (TMAH) having a concentration between 25 wt% and 15 wt% at 80 degrees C. Etch anisotropy, as seen in the under-etched inclined planes and in fast and slow etching planes, is found to be quite different in the two cases. In 25 wt% there is a deep local minimum in etch rate at the {100} planes, while in 15 wt% there is a shallow local minimum at the {110} planes. In between these two concentrations, more complex behavior is observed. The addition of isopropyl alcohol to the TMAH etchant further changes the etch anisotropy.