Flat washers feature a hole in the center matching the bolt they're used with. Locking washers, equipped with recessed features, prevent loosening. Some types include split lock washers, which use compression to prevent loosening; wave washers with a wavy surface for grip; and spring washers, which absorb vibration and shock. SAE/USS washers are designed for SAE (US) or USS (UK and others) fasteners. Finally, machine screw washers pair with machine screws to fasten metal pieces together.