A stainless washer to suit 5/16 inch bolts and cap screws. Fitting one under the nut, and under the head where the surface is soft, spreads the clamping load across a wider footprint so the fastener does not pull into aluminium, thin steel sheet or composite panels. That matters for keeping a joint tight over time, because a fastener that embeds itself into soft material will lose preload and work loose. The washer also gives a smooth face for the nut to turn against, so your torque translates into genuine clamp load. Stainless construction avoids rust marks running down painted or polished panels, which is worth having on any visible or weather-exposed fastener.