Flat washers are used to strengthen & reinforce metal, wood or other material at the point where fasteners pass through. Lock washers are split and curled slightly so when the fastener is tightened, the curve keeps tension against the nut or bolt-head to help prevent it loosening from vibration, movement or variable strain. Flat washers are often used with lock washers to sandwich the lock washer between the flat washer and nut or bolt. Set includes: 50pcs #6 flat washer, 50pcs #6 lock washer, 40pcs #8 flat washer, 40pcs #8 lock washer, 30pcs #10 flat washer, 30pcs #10 lock washer, 25pcs flat washer, 25pcs lock washer, 15pcs 5/16 flat washer, 15pcs 5/16 lock washer, 15pcs 3/8 flat washer and 15pcs 3/8 lock washer.