Abstract: An improved lock washer locatable between the head of a fastener bolt and e work being fastened. The lock washer includes a retractable blade having a "hinged" connection with the main plate area of the washer. During thread-in operations the blade assumes a non-obstruct position. When the bolt is torqued to its seated position the blade is engaged with a shoulder formed on the head of the bolt, thereby positively obstructing the bolt from unthreading movement. If it should be necessary to unthread the bolt the blade can be manually pried or forced to a non-obstructing position. The blade has a positive lock action that does not depend on frictional forces.