The element on the end of a drill pipe that actually does the cutting (see Figure D-26). Drill bit diameters are often 12.25 in. (31 cm), 8.5 in. (21 cm), or 6 in. (15 cm). A drag bit or fish-tail bit has teeth that cut and tear soft formations as the bit is rotated. A roller bit has hardened teeth on wheels that alternately put formations under pressure and relieve the pressure, which causes rock pieces to flake off. A diamond bit uses industrial diamonds set into the cutting surfaces to drill very hard formations. Core bits are designed to retrieve rock cores.