Flat washers are essential for adding resistance to bolts or screws, preventing them from digging into surfaces and causing damage. Positioned behind a nut or screw head, they help distribute the load evenly and prevent material buckling or screw pull-through. Washer sizes correspond to the diameter of the bolt or screw they accompany and should be properly matched to ensure an effective fit. When choosing a washer, it's important to select one that complements the finish of the fasteners in use, as differing finishes can lead to chemical reactions that weaken the fasteners over time.