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I see people doing this a lot:

enter image description here

My question is: Why?

When is it necessary to double up locking carabiners? Ever? Doubling up non-locking carabiners with opposite and opposing gates was, and still is a safe practice, but locking carabiners were an innovation invented to provide the same security, but eliminate the necessity of using two carabiners. Yet, there are people doubling up locking carabiners, as if they didn't have locks on them at all, and recommending it as best-practice. I support the notion that there is no such thing as overkill, but, if you're using a 25kN biner with a triple action gate, you are going to be plenty safe top-roping or using the biner to tie-in to your harness. So why are so many people strict on the practice of doubling up on lockers?

ShemSeger
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    Are you referring to an anchor master point? Top rope and/or belay anchor? – Chris Mendez Jan 31 '16 at 13:43
  • See this answer for a PDF by Petzl that advises to always use two locking biners to attach a rope to the harness: http://outdoors.stackexchange.com/a/10747/5657 – anderas Feb 01 '16 at 17:50
  • I do think it could vary on the particular usage. There are so many sports/activities/disciplines/jobs... where carabiners (locking/non-locking) are used. Are you referring to specifcally in rock climbing I take it? – montane Feb 02 '16 at 23:50
  • I spotted an relevant anecdote on MountainProject this afternoon: "I had a rope unclip itself from one of the draws and we couldn't tell until someone reached the anchor. Basically the rope had twisted but the twist was out of sight, when unweighted it went to twist back, laid across the gate, was weighted and unclipped." In this situation I can also see the rope also unscrewing a locker or similarly opening a triple-action gate. – requiem Feb 03 '16 at 02:55
  • @requiem Yup, there's a reason why I'm slowly replacing all my screw gates with double gates. But I'd be willing to bet money you couldn't open a triple action gate with the rope. I have to struggle sometimes just to get some of my triple action gates to open using two hands. – ShemSeger Feb 03 '16 at 03:18
  • @ShemSeger It does need a bit of tension on the rope to easily replicate, but here you go: https://dl.dropboxusercontent.com/u/25884063/triple-action-locker.m4v – requiem Feb 03 '16 at 07:20
  • @requiem, I don't think that counts, he had to hold the carabiner firm in his other hand in order to do that trick. I still seriously doubt that could happen while clipped to either the belay loop of a harness or a master point. – ShemSeger Feb 03 '16 at 16:22
  • Everyone is just thinking about climbing. I couldn't find an answer or a comment about mountaineering. But there are a realy important reason for two locking biners. Just think of walking in a roped team. Due to walking (and all those other movements and vibrations) your biner COULD unlock. That's why I use two locking biners for my teams to clip in the rope on glaciers. I don't want to remind them 'check your biners' every 5min. – Phab Feb 05 '16 at 07:48
  • As you can see in the next picture, sometimes people forget to lock them. – StrongBad Feb 05 '16 at 14:32
  • @Phab, Gridlock, magnetron, locksafe, and twin gates are all superior to screw gates, and you don't have to keep checking them to ensure that they are still locked. One of either of the above is cheaper than two screw gates as well. – ShemSeger Feb 05 '16 at 19:28
  • @ShemSeger Sure, but the picture in the question shows two screw gates. Also all gates you mentioned have pros and cons, like handling with gloves. But discussing about this is not the topic I think. – Phab Feb 08 '16 at 06:36

3 Answers3

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A number of prominent climbing organizations (e.g. International Federation of Sport Climbing) either recommend or require two locking carabiners for clip-in attachment to a harness, e.g.

  • IFSC Rules 2015 [1MB DOC]

    8.3.5 The climbing rope shall be connected to the competitor's harness by two Screwgate or Self-Locking Karabiners arranged in opposition (i.e. with the gates in opposition) and the climbing rope must also be attached to the Karabiners using a “figure of eight” knot, secured with a “stopper” knot or tape.

  • USA Climbing Rulebook [1MB PDF]

    4.1.11 USAC recognizes that some host facilities utilize a pre-tied rope as a method to tying-in. In this case, it is recommended that two opposite facing auto-locking carabiners, at the tie-in point, be used.

Some organizations feel that even two opposite-and-opposed screw-gate carabiners are insufficient, such as the Australian Professional Association of Climbing Instructors (PACI), who released a detailed paper on their recommended protocol:

Their protocol requires additional measures such as captive-eye carabiners, the reasons for which are detailed within that paper.

For other uses I found a couple of relevant statements:

  • The American Alpine Institute blog with regard to the "power point":

    In the guiding world, two opposite and opposed lockers are considered to be industry standard. The liklihood of a single locking carabiner becoming unlocked and opening is incredibly low. However, this is one of the rules that you learn when you start to climb and it has become so integral to outdoor groups throughout the world in toproping that it has become the industry standard across the board.

    ...

    We do teach to use two lockers on a belay loop for tying in for glacier travel.

  • From Rock Climbing: The AMGA Single Pitch Manual

    In the professional realm, the industry standard for attaching the climbing rope to the toprope anchor master point is either two locking carabiners or three oval carabiners with the gates opposed and reversed.

Mr.Wizard
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    I like the PACI protocol, it makes the most sense, they cut straight to the issue that a standard carabiner connected to your harness isn't guaranteed to stay in the proper orientation, and could easily rotate the load the gate. Good research. – ShemSeger Feb 01 '16 at 03:06
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It's not necessary, but neither is using two non-lockers really.

People usually use two carabiners because they will bend the rope less sharply, reducing friction when loaded (also reducing wear on the biners). This is very common in top roping, when the climber is expecting to be lowered.

I suspect the person that setup the anchor in the picture wanted more smoothness, and left one of the carabiners unlocked just to be cool.

Pepi
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    This is not correct in quite a few cases. For example, on topropes or tie-in points, you definitely wouldn't want use a single non-locker. In case of a fall, a non-locker could easily snap open for a split second, for example by hitting the rock. That's why you would use two of them, so that at most one snaps open and the other one keeps you safe should the rope/harness wiggle out. – anderas Feb 01 '16 at 17:46
  • I agree that having double carabiners increases the radius which is nicer on the rope and the carabiners. – Erik Feb 04 '16 at 18:27
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For my uses {natural fibers, normally) , having a wider contact surface, a gentler angle of ascent and descent as the rope slides is a factor. As stated above{ " bendng the rope less sharply "} A specialized aspect but important to my segment of the community.

James Patrick Hurley
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