A carabiner is a specialized type of shackle, a metal loop with a spring-loaded gate used to quickly and reversibly connect components, most notably in safety-critical systems. Carabiners, often called D-Rings by military professionals, are widely used in rope-intensive activities such as climbing, arboriculture, caving, sailing, hot air ballooning, rope rescue, construction, industrial rope work, window cleaning, whitewater rescue, and acrobatics. They are predominantly made from both steel and aluminum. Those used in sports tend to be of a lighter weight than those used in commercial applications and rope rescue. Often referred to as carabiner-style or as mini-biners, carabiner keyrings and other light-use clips of similar style and design have also become popular. Most are stamped with a “Not For Climbing” or similar warning due to a common lack of load-testing and safety standards in manufacturing. While from an etymological perspective any metal attaching link with a spring gate is technically a carabiner, the strict usage among the climbing community specifically refers only to those devices manufactured and tested for load-bearing in safety-critical systems like rock and mountain climbing.
Life-supporting carabiners such as those used in tree climbing need to be strong (thus strength ratings), but also secure against unintentional opening underuse. All carabiners with a spring-loaded gate “are self-closing” (single action). Several are also “self-locking” (double action), some even “self double locking” (triple-action).
Locking carabiners have the same general shape as non-locking carabiners but have an additional mechanism securing the gate. These mechanisms may be either threaded sleeves (“screw-lock”), spring-loaded sleeves (“twist-lock”), magnetic levers (“Magnetron”), other spring-loaded unlocking levers, or double-gates (“Twin-Gate”).