Plain washers, also known as flat washers, are the most common type of washer. Plain washers do not have a locking function, rather they are used to protect the mating surface from damage. They do this by increasing the surface area in order to help distribute the load, in particular on soft materials or over large or slotted holes. Plain washers are installed between the bolt head/nut and the mating surface. The bolted joint is then tightened in the same way as an unsecured bolt.