A flattened, ring-shaped device used to improve the tightness of a screw fastener. Three types of washer are in common use: plain, spring-lock, and antiturn (tooth-lock) washers. Standard plain washers are used to protect a part from damage or to provide for a wider distribution of the load. Because a plain washer will not prevent a nut from turning, a locking-type washer should be used to prevent a bolt or nut from loosening under vibration (see illus.). For industrial applications, spring-lock washers are intended to compensate for possible loosening between assembled parts and to facilitate assembly and disassembly. Lock washers create a continuous pressure between the parts and the fastener. The antiturn-type washers may be externally serrated, internally serrated, or both. The bent teeth bite into the bearing surface to prevent the nut from turning and the fastening from loosening under vibration. To speed up assembly, a variety of permanent preassembled bolt-and-washer and nut-and-washer combinations are available.