A taper washer is used to create a mating surface perpendicular to a fastener, for example if a wedge were to be bolted to a flat plate a taper washer would be used to provide a perpendicular surface under the nut (or the screw head). Without the taper washer, the claming force would all be on one side of the nut, interfering with tightening and bending the shaft of the fastener instead of stretching it. A tapered washer is a wedge in a plane perpendicular to the axis of the through hole, i.e. its thicker on one side than the other. The taper is chosen to match the application.