Miniature and instrument ball bearings come in two basic types: deep groove ball bearings and angular contact ball bearings. Deep groove ball bearings are further divided into five types based on detailed design: standard type, flanged outer ring type, wide inner ring type, expanded type with radial thickness of the outer or inner ring much greater than the standard bearing width, and thin type with very thin radial dimensions of both inner and outer rings. Deep groove ball bearings can also be categorized by sealing or dust-proofing into "open type," "dust cover type," and "sealed ring type."