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A carabiner (//) or karabiner is a metal loop with a spring loaded gate used to quickly and reversibly connect components in safety-critical systems. The word is a shortened form of "Karabinerhaken", German for "spring hook for a carbine".
Carabiners are widely used in rope-intensive activities such as climbing, arboriculture, caving, sailing, rope rescue, construction, industrial rope work, and window cleaning. They are made from both steel and aluminium. Those used in sports tend to be of a lighter weight than those used in commercial applications and rope rescue. Carabiner-style keyrings have also become popular, most stamped with a "Not For Climbing" warning.
Physical properties 
Carabiners come in four characteristic shapes:
- Oval: Symmetric. Most basic and utilitarian, also the least expensive. Smooth regular curves are gentle on equipment and allow easy repositioning of loads. Their greatest disadvantage is that a load is shared equally on both the strong solid spine and the weaker gated axis.
- D: Asymmetric shape transfers the majority of their load onto the spine, the carabiner's strongest axis. Slightly more expensive.
- Offset-D: Variant of a D with a greater asymmetry, allowing for a wider gate opening. Still more expensive.
- Pear/HMS: Specialized oversized offset-D's used in belaying. Most expensive and heaviest carabiner.
Locking mechanisms 
There are two broad categories of carabiner, locking and non-locking.
Non-locking carabiners have a sprung swinging gate that accepts a rope, webbing sling, or other hardware. Rock climbers frequently connect two non-locking carabiners with a short length of nylon web to create a quickdraw.
Three gate types are common:
- Straight gate: The most utilitarian, and hence most popular.
- Bent gate: Curved gates allow for easier clipping in and out in special situations, such as connecting a rope to a quickdraw. Gate strength remains on a par with straight-gate carabiners.
- Wire gate: The lightest type, with a strength roughly equal to the others, allowing more to be carried for a given weight. Wire gates are less prone to icing up than solid gates, an advantage in Alpine mountaineering and ice climbing. The reduced gate mass makes their wire bales less prone to 'gate flutter,' a dangerous condition created by irregular impact forces generated by the climbing rope or contact with hard surfaces in a fall which momentarily opens the gate (and both lowers the breaking strength of the carabiner when open and potentially allows the rope to escape).
Locking carabiners have the same general shape as non-locking carabiners but have an additional sleeve securing the gate. These sleeves may be either threaded ("screw-lock") or spring-loaded ("twist-lock").
- Screw-lock: Have a threaded sleeve over the gate which must be engaged and disengaged manually. They have fewer moving parts than spring-loaded mechanisms, are less prone to malfunctioning due to contamination or component fatigue, easier to employ one-handed. They, however, require more total effort and are more time-consuming than twist-lock.
- Twist-lock: Have a security sleeve which must be manually rotated to disengage, but which springs closed automatically upon release. They offer the advantage of re-engaging without additional user input, but being spring-loaded are prone to both spring fatigue and their more complex mechanisms becoming balky from dirt, ice, or other contamination. They are also difficult to engage one-handed and with gloves on.
- Recreation: Carabiners sold for use in climbing in Europe must conform to standard EN 12275:1998 "Mountaineering equipment - Connectors - Safety requirements and test methods," which governs testing protocols, rated strengths, and markings. A breaking stress of at least 20 kN (20,000 newtons = approximately 4,500 pounds of force which is significantly more than the weight of a small car) with the gate closed and 7 kN with the gate open is the standard for most climbing applications, although requirements vary depending on the activity.
- Industry: Carabiners used for access in commercial and industrial environments within Europe must comply with EN 362:2004 "Personal protective equipment against falls from a height. Connectors." The minimum gate closed breaking strain of a carabiner conforming with EN 362:2004 is nominally the same as that of EN 12275:1998 at around 20 kN. Carabiners complying with both EN 12275:1998 and EN 362:2004 are available.
United States 
- Fall protection: Carabiners use for fall protection in US industry are classified as "connectors" and are required to meet Occupational Safety and Health Administration standard 1910.66 App C Personal Fall Arrest System which specifies "drop forged, pressed or formed steel, or made of equivalent materials" and a minimum breaking strength of 5,000 lbf (22 kN).
American National Standards Institute/American Society of Safety Engineers standard ANSI Z359.1-2007 Safety Requirement for Personal Fall Arrest Systems, Subsystems and Components, section 188.8.131.52 (for snap hooks and carabiners) is a voluntary consensus standard. This standard requires that all connectors/ carabiners support a minimum breaking strength (MBS) of 5,000 lbf (22 kN) and feature an auto-locking gate mechanism which supports a minimum breaking strength (MBS) of 3,600 lbf (16 kN).
- Rescue: Minimum breaking strength requirements and calculations for rescue carabiners are set out in National Fire Protection Association standard 1983 Fire Service Life Safety Rope and Equipment. The standard defines two classes of rescue carabiners. Light use rescue carabiners are required to have minimum breaking strengths of 27 kN gate closed, 7 kN gate open and 7 kN minor axis. General use rescue carabiners are required to have minimum breaking strengths of 40 kN gate closed, 11 kN gate open and 11 kN minor axis. Testing procedures for rescue carabiners are set out in ASTM International standard F 1956 Standard Specification of Rescue Carabiners.
See also 
- "Climbing Dictionary & Glossary". MountainDays.net. Retrieved 2006-12-05.