Nuts are climbing protection, used predominantly when trad climbing. There are many varieties available from different manufacturers, however the basic format is a small wedge shaped block of metal with a loop of wire attached to it. The nut is placed into a narrowing crack in the rock face and wedged so that it will not be removed in the event of a fall. The climber will then attach a quickdraw to the loop and clip their rope through the other side of the quick draw. In the event of a fall, a well placed nut should hold fast and prevent the climber from falling any further.
Nuts are available in different sizes and are usually colour coded either by anodising the block or a small collar on the wire. The colour represents the size of the nut, which after a little time using them will help to speed up the process of which nut to select whilst climbing. Nuts are also known as wires or stoppers and although each manufacturer seems to have a different name for them, they all perform in roughly the same way.