Carabiners are used to clip slings, cord, the rope, and anything else. Carabiners have multiple kN strength ratings for the axis of load. Climbing carabiners should be rated to at least 20kN on their strong axis and 7kN on the short axis according to ASTM Standard F1774. They are strongest along their spine, and significantly weaker along their other axis or when open.
Locking carabiners should be used whenever a person or the rope is involved (e.g. belay device, attaching yourself to an anchor, rappel third hand, top-rope anchor).
Cross Loading
Care should be taken to never cross load a carabiner. Cross loading is when a carabiner's spine is levered against a rock or something else so that the load threatens to break the spine.
Carabiners come in many shapes and sizes. Smaller carabiners are often lighter but harder to clip the rope through. Oval carabiners are symmetric and are generally heavier for a given strength, but useful for even load. D carabiners and asymmetrical carabiners concentrate the load on the spine of the carabiner where the carabiner is the strongest.
