Nuts are a form of traditional rock climbing protection. The nut is a piece of metal attached to a wire. The nut is placed in a constriction in the rock, and the wire is clipped with a quickdraw.
A good nut placement has good surface area contact on both sides and fits well into the constriction. It should also resist a moderate outward pull (such as might be generated by the rope bouncing around or during a lead fall) in addition to holding a strong downward pull.
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