In rock climbing, a copperhead is a small nut made of a soft metal, originally copper or brass, now usually aluminium. Copperheads are placed in the smallest cracks and seams where their malleability means that they can conform to the rock and grip better: often they need to be hammered into place. Their small size and low strength makes them among the poorest kinds of protection; their main use is in aid climbing where a placement that will just support the weight of the climber can be used to make progress, even though it would be useless in a fall.