Many caves have been equipped with bolt holes consisting of internally threaded metal inserts fixed into holes drilled in the rock. Subsequent visitors carry their own belay eyelets to bolt into these points. Selecting the best position for bolts needs particular care, the aim often being to achieve a 'Y' shaped rope hang between two bolts on opposite walls. This technique can help to prevent the rope from coming in contact with the rock, shares the load between the two belays, and reduces the shock load should one fail (in comparison with the use of a slack backup rope).