It's a loop knot made using a figure-8, then retracing the knot with the tail. Climbers use it where sailors would tend to use a bowline. I've heard various reasons why: "it's easier to teach people in the gym", "it's harder to get wrong", "the dynamic ropes climbers use put bowlines at risk of capsizing".
1. It's very easy to visually check (five pairs of parallel lines) (by easy I don't just mean reliably, I also mean low effort, which is important because you do it a lot - point fingers at the pairs of parallel lines: "two four six eight ten"). It's very symmetric. If done wrong it tends to look radically different.
2. It requires pulling in 2 places in different directions to come undone. The bowline opens if you pull in 1 bad spot.
(3. In the gym, you can just leave the normal figure eight on the rope, which saves the next climber a bit of time. Not a lot, but it's a nice feature.)
Nobody does a bowline, that's insane because you can't ring-load (cross-load) it. [0]
Europeans sometimes do the double bowline (traced), but the naming is ambiguous in english, so you have to google double bowline on a bight.
Americans used to do yosemite bowlines, but that fell out of fashion because figure eight (safe and foolproof) and double bowline (safe, trivial to untie after hefty lead falls) cover literally every use case perfectly.
I've been using a double bowline with the Yosemite-style stopper for a long time (I've also heard it referred to a Jack's bend, but I can't find anything quickly to confirm that). Some people give me a hard time about using a double bowline, saying it is "unsafe," but their reason usually boils down to "it's hard to visually inspect." My belay partners (very small list, because I don't let just anyone belay me on lead) typically remind me to re-check my knot just before I leave the ground.
It's generally agreed that it is totally safe (it solves the crossloading issue). I think we can all stick with the proven safe knots that we are comfortable with, and the fact it makes partner check a bit trickier applies also to the double bowline on a bight. I'm equally happy to triple-check myself and make it clear to my belayers that they are off the hook there.
Here is my pitch for "doppelter bulin" (double bowline on a bight):
Even if by some miracle the ENTIRE retraced half of that knot came undone, you would STILL be tied in with a normal bowline. I do a stopper knot too, but I think that illustrates just how bomber it is. And I only use it for single pitches for lead falls, on multipitches I still simply do a figure 8, because why get fancy?
A bowline with stopper is a pretty common climbing knot in the UK [0]. I don't have the expertise to weigh into any debates about how safe it is, but I've seen a lot of people use it without incident, and there's no question it's easier to untie than a rethreaded figure-eight.
The yosemite variant is fine, but maybe we shouldn't take the world's best climber as a reference who has a lot more context and experience to make very measured tradeoffs:
> IMPORTANT: This knot is methodically NOT recommended and yes, it MAY untie while you are climbing. It can happen if your rope is new-ish (and that means it slides easily) and if you don’t tighten the knot with a lot of force. I am always splashing chalk on the knot in case of new rope to increase the friction and I do tighten it with a lot of force. The reasonable and recommended alternative, pretty common and very safe is double bowline. It is almost as big as eight, but at least it is always easy to untie.
My understanding why bowlines are disliked in climbing community is that it may fail if the pull does not come from the long tail, but from inside the loop. That is, cliiping two carabiners to bowline loop and pulling the carabiners apart is not a safe way to use bowline.
I've been sailing since I was seven years old, and I still have to think when I'm tying a bowline. So I can believe the comment about it being easier to teach.
The only real difference is the trace version you can tie into an existing loop where with the bight version you need a carabiner to connect in. Every time I've done climbing or repelling we've just used carabiners instead of tying directly into the rope.
The more correct term would probably be a figure 8 on a bight. But since climbing harnesses have permanently sewn loops to tie into you can't actually tie it on a bight. You retrace the knot. Otherwise it's a standard figure 8 knot with a loop coming out one end.
I don't think theres any difference in the final knot - its how you tie it. Tying a figure of eight in the rope and then re-tracing it back is necessary because your harness loops (not shown in the pic) will be in the bight. I'm guessing a sailor wouldn't need to tie anything into the bight, so retracing wouldn't be necessary.
I've never heard of a fig-eight being referred to as "trace" but this may be an US-ism and I'm European.
You can tie a trace 8 directly into an existing loop like your climbing harness where an 8 on a bight needs a carabiner. Most people would just use a carabiner anyways though.
