crystalline structure of Si wafers used for MEMS
The flats on Si wafers are not arbitrary.  The largest, or primary flat, is perpendicular to the &lt;110> orientation in the plane of the wafer. If it exists, the secondary flat helps identify the wafer orientation and doping type (and can easily vary based on wafer maker and the company ordering the wafers). All modern wafers (anything 200mm or bigger, almost all 150mm) are (100) and boron doped, so the secondary flat isn't used.

The primary flat has a specific orientation since the chips need to be aligned along the &lt;110> crystallographic orientations so the wafer can be diced up.  This usually uses a diamond scribe and careful breaking rather than sawing, because the saw is wider than a scribe line (so fewer chips/wafer) and dirtier .

So, you wafer has &lt;001> pointing up, and &lt;110> in plane towards the flat.