Climbing Nut (also known as a chock). The climbing nut (see schematic below), is an asymmetric piece of metal through which a metal loop of wire is threaded. When climbing the climber looks for a crack in the rock which narrows downwards. The nut can be slotted into the crack from the top and hopefully the nut will jam in the crack. The climber clips a carabiner through the loop in the wire and the rope through the the carabiner.

This piece of climbing equipment originated from small pebbles that climbers in the 1930's would carry with them. They would slot the pebble into the crack and tie a piece of rope around the pebble and the rope they were climbing on. This was not a very safe piece of equipment

Post war climbers soon developed more sophisticated equipment such as that shown below (yeah really sophisticated huh?). These pebbles were originally called chock stones which is where the alternative name for the nut comes from.

Several climbing companies manufacture their own line of nuts, notably DMM, Wild Country and Black Diamond. They come in a range of sizes generally from 00 - 10. A number 10 is about half the size of a fist, a 00 is about the size of your little fingernail.

After some time climbing one can look at a crack and recognise which sized nut will fit into it. Before this skill is developed a lot of time can be wasted "fiddling with your nuts". The second's most important job is to remove the nuts from the cracks that the leader has placed.

                      insert nut
side view
   ------          \          |
   \  _ |           \         |
    \ \||           \         |  ----
     \  |            |        |   crack in 
      \_|            |        |    rock
       I             |        | 
       I             |        |   (looking
       I              |       |     face on)
       I             \        |
       I              |       |   
       I              \       |
       I               \      |
       I                \     |
       I                 \    | 
      I I                   ^
      I I.                 nut 
      I I                will wedge here
                                  V direction 
front view                            of gravity
    |     |
    |     |
    |     |  -- metal head
       I                           ^
       I                         direction of
       I                            climber
       I  -- metal wire             (mostly)
      I I
      I I.  --loop through which crab is clipped
      I I

top view

    ------                         A       --
   |     |                          G     |  climber who  
   |     |                           H    V  didn't know 
    ------                            h      how to place
                                       h      a nut.