Flat

Flat washers are the most common, with two flat sides. They are circular and come in a variety of sizes and thicknesses.

Flat washers are general purpose, used to distribute force, protect the item to which it's attached and to correct the size of an opening.

Lock

Lock washers are typically made of some type of metal and come in either a split, coiled or star shape.

Lock washers are used to prevent the bolt or fastener from loosening, effectively "locking" the bolt in place.

Conical

Also known as a Belleville or spring washers, conical washers are flat washers pressed into a conical shape.

Used in higher-friction applications to absorb vibration and shock, such as in cycling. They can act as a spring when stacked together in different configurations.

Countersunk

These metal washers are raised on the sides and are flat in the center.

Countersunk washers are used with a flat- or oval-head bolt or screw so they can sink into the center, allowing for a more finished appearance.

Insulator

Also known as shoulder washers, insulator washers are nonmetal, typically made of nylon with long or short barrels.

These washers act as an insulating seal to block electrical currents and protect fasteners from moisture and dirt in electrical applications. They can also be used to reduce vibration.

Retaining

Also called push nuts, retaining washers come in a variety of shapes and dimensions and can be made of nylon, zinc, steel or black phosphate. 

Effective at securing assemblies in place, these are often used in environments subject to vibration or corrosion, e.g., automotive applications.

Rivet

Rivet fasteners can be round or square, and they are made of steel or aluminum. Rivet fasteners are used exclusively with blind rivets.

Effective for fastening to soft or brittle materials, rivet fasteners are also useful with oversized openings.

Sealing and Cushioning

Sealing and cushioning washers are round, and often made of softer materials.

These washers are used to form a seal around a bolt or fastener. They are sometimes used to bond to metallic washers and are also used to reduce noise and vibration.

Spherical

Spherical washers feature a two-piece equalizing assembly with one flat washer and one dish-shaped washer.

Spherical washers are used to eliminate unequal thread loads and stress from misaligned parts and nonparallel surfaces.

Square

Square washers are flat washers in a square shape. They have a larger surface area than round washers for better torque distribution. They are thick with a broad shape to protect against the damage of wood fibers used in wood construction.  Square washers can be made of a variety of different steel types including stainless, galvanized, zinc plated and black oxide.

Square washers are used to prevent rotation and aid in corrosion resistance. They are also used to limit sound levels, vibration, and electrical current for use in seismic applications.

Structural

These can be round or square, with the top-bearing surface cut at an oblique angle to compensate for nonparallel surfaces.

Used in structural leveling and shimming applications.

Wave

Curved in two directions like a wave, these washers are usually made of metal.

Used as spacers or cushions. They act as a spring when compressed to maintain fastener tension and sustain loads. Used in low-strength fastening applications.