Nuts and bolts come in various sizes and head styles to suit different purposes. In nearly all situations, the nut and bolt is used with at least one washer. Different washer types are used depending on the application and materials bolted together.
Lock washers are cut and slightly bent. Their main purpose is to "lock" the nut in place by exerting an uneven force on the face of the nut. Lock washers are often used in addition to standard washers.
The main purpose of a flat washer is to provide a larger surface for a bolt to exert pressure. Used on wood or other soft materials, flat washers help to keep bolts from pulling through material.
The correct size washer should be used with a nut and bolt. The washer should carry the same size designation as the bolt, such as 1/4 inch or #10. Using a washer that is too large may cause the bolt or nut to pull through the washer.
Speciality washers are used with different materials for different purposes. They might be spacers or special-purpose locking washers.
Lock washers are placed closest to the nut, while flat washers are closest to the material being bolted. Adding a second machine washer between the bolt head and the material is a good way to add strength.