Conventional rotary drilling is used for the bulk of oil and gas well drilling. Most drilling systems are built around the concept of rotating the bit on the hole bottom and circulating a drilling fluid. Until the early 1980s, when PCD bits were introduced, the common types of bit were milled tooth, insert and surface-set natural diamond. PCD bits are designed to remove the rock by shearing rather than by crushing or grinding, as is the case with roller cone bits. This is possible because PCD combines the hardness and abrasion resistance of diamond with the impact resistance of tungsten carbide.