Micro and instrument ball bearings are divided into two basic types: deep groove ball bearings and angular contact ball bearings. Deep groove ball bearings can be further divided into the following five types based on detailed design: standard type, flange outer ring type, wide inner ring type, extended type with outer or inner ring radial thickness much larger than that of ordinary bearings, and thin type with both inner and outer rings radially very thin. Deep groove ball bearings can also be classified into "open type", "dust cover type", and "sealing ring type" according to their sealing or dust-proof forms.