Up to the 150 mm wafer diameter era, wafers had flats, and the flats indicate the following information:

1. The doping type of the wafer (n- or p-type)
2. The orientation of the wafer: {100} or {111}
   
   And wafers with diameters larger than 200 mm generally will have no flat at all, but just a small "notch" as shown in Fig. 3.9.
   
   Fig. 3.9 Identification of wafers