A washer is a metal or non-metal material that is made to go underneath a nut, bolt, or fastener. Mainly used to stop a leak or simply stopping something from vibrating loose. In addition to that they may also be used to help disperse the amount of pressure exerted onto a hole or joint. Lock washers are made for fasteners that tend to loosen from vibration or friction. They are many different types but they all try to achieve the same goal, and that is to lock a fastener in place. Washer dimensions can come in either imperial or metric units. If you do not have a specific size most manufacturers will be able to custom build a washer to fit your application. Washers can follow ANSI, SAE, USS, and JIS sizes as well.