Washers distribute the pressure of a fastener head evenly over a surface to reduce loosening over time. They ensure that the fastener is pressed against a smooth surface to prevent damage to the material. Flat washers are the most common type of washer with completely flat surfaces for distributing the force of a nut or bolt. Lock washers secure nuts and bolts in place by biting into fastener heads and materials to create tighter assemblies than flat washers. Sealing and cushioning washers form an airtight seal between fasteners and surfaces to prevent fluid leakage. Countersunk washers have a recessed, bowl-shape so screws lie flush with the work surface to create a finished, smooth look.