Normally, plain and lock washers are used in aircraft structures. Plain washers are made of carbon steel and are used for nuts to protect the metal from being scratched when the nut is turned down. The plain washers also may be used as a shim under a nut to compensate for a bolt that is slightly long. Plain washers come in two sizes: AN 960-616, which is the regular washer that fits a 3/8 bolt, and AN 960-6161, which is the light series that fits the same bolt. AN stands for Army-Navy. Lock washers come in various designs. These are used to hold the nut under tension on a bolt to prevent its turning. The most common of these is the split lock washer (AN 935) made of heavy spring steel, split, and twisted. AN 936 lock washer made of much thinner spring steel, comes in two varieties. One has teeth cut into the inner circumference; the other has its teeth in the outer circumference.