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As far as I understand:

  • When you're climbing, you tie your rope through your harness' 2 tie-in points, using a figure 8 knot.
  • When you're belaying, you tie your GriGri or ATC to a carabiner that goes through the belay loop, which in turn goes through the tie-in points.

I have two questions:

  1. Why do these attach to different places? I.e. Why doesn't the rope go on the belay line, or the GriGri/ATC on the tie-in loops?
  2. It looks like the belay loop is a single point of failure. While I'm sure it's a strong single point failure, it's still... just that. How come they aren't redundant like the tie-in points?
  • "you tie your GriGri or ATC to a carabiner" You connect it to the carabiner, you can't tie a carabiner. – endolith Apr 16 at 19:28
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It does depend on the type of harness. Some don’t have separate tie-in points, particularly entry level harnesses used by indoor walls.

Harnesses are built to be bomb-proof where it matters and a well looked after harness with no signs of damage or wear will never, ever, break at the belay loop or tie-in points.

The main points of the two tie-in points apart from the redundancy, is that (I believe) in the event of a big lead fall they will spread the impact of the fall across your body rather than in one spot. If you miss tying into both and only tie into one, or tie into the belay loop instead, you will not be at significantly more risk of the harness breaking. There is some discussion (and mention of the infamous Todd Skinner case) in this article.

The point of the belay loop is to allow a carabiner to be orientated correctly when belaying. Redundancy in the loop isn’t necessary if you are belaying. If your climber falls there is less impact on your belay loop as you are static and a lot of the force of the fall is taken by the rope, other gear in the system, and also lessened by good, dynamic belaying techniques.

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  • "The point of the belay loop is to allow a carabiner to be orientated correctly when belaying." Ah, this makes sense. Still surprises me that there aren't 2 belay loops, however. "a lot of the force of the fall is taken by the rope" Is this because of the slight deformation of the rope? In a sense, it's a dampening system that reduces the peak force felt on the belay's side? – Alexander Jun 27 at 1:36
  • @Alexander-ReinstateMonica Yes, the stretching of the rope absorbs the force of the fall. – Darren Jun 27 at 8:05
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Most harnesses allow you to either tie into your belay loop or through the two loops the belay loop goes through. This does not really matter. The belay loop is tested with 15kN in the norm: UIAA harness norm
(Source: UIAA)

A load of 15kN is by far exceeding any load that could ever be applied. While the norm specifies a maximum of impact force of 12kN for single ropes, this is with a high fall factor, static belay and a metal mass instead of a human body. A more realistic maximum value for climbing is considerably lower.

The Petzl experiment also shows that the forces for the belayer do not increase when increasing the fall factor from 0.7 to 1, they are limited to about 2kN. At some point the belayer (if not anchored) is lifted off the ground and won't be able to apply more force on the belay end.

Looking at the numbers, we see that while the belay loop is a single piece failure, it is also considerably over-engineered in terms of load it can take. For belaying we are talking about a factor of 7-8

The real reason not to tie into the belay loop with the rope is that its more convenient when using double (half) ropes. If your climb is zig-zagging along a bit and you use one rope for the left side and one for the right side, it is very convenient to have these ropes left and right of your belay loop.

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  • Good answer, but I'm not sure what you mean by your last paragraph. Do you mean you tie twin ropes one onto each tie-in point? – Darren Apr 16 at 11:15
  • Maybe the term double rope is not really chosen well here. Typically double ropes mean half ropes (a single strand may be clipped into a piece) as opposed to twin ropes (always clip both strands into each piece). With twin ropes you can also tie in left and right but you want get the advantage as you always clip both – Manziel Apr 16 at 11:23
  • I understand that belay loops are over-engineered given their important role. But nevertheless, manufacturing defects happen (rarely, I hope!). Even if they have a defect rate of say 0.01%, doubling up would be a cheap way to reduce the rate to 0.000001%. – Alexander Jun 27 at 1:39
  • Manufacturing defects happen but rarely. Any manufacturer with a defect rate of 1 in 10 000 units sold (=0.01%) would soon be out of business. Also most defects do not mean that a piece does not function at all but rather that it breaks at a lower load (e.g. 10 kN instead of 15 kN). Doubling the belay loops addresses a negligible risk while creating a whole new spectrum of problems such as people tying into their loops incorrectly, using only one loop, etc. – Manziel Jun 30 at 7:35
  • To illustrate the problem of users further...there is quite a number of accidents because people use their gear loops to tie in instead of the belay loops (while I personally have never heard of belay loop breaking on a newish harness). For a short time there have been attempts by some manufacturers to solve the problem by making gear loops load-bearing. This created even more problems as harnesses started to have different characteristics by manufacturer and people started to tie into their gear loops which was "safe" on some harnesses and outright dangerous on others. – Manziel Jun 30 at 7:42
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Other answers are good and one thing I would like to mention is ergonomics.

Thing about which way the grigri would face if was also clipped into the tie-in-points. The grigri would be sideways (that is to saw the rope would feed out horizontally). This yields different body mechanics. Back in the day when we would hip belay that motion made sense since when you would arrest a fall your hand can drop to the opposite hip. Using an ATC in this way is actually still somewhat common.

I would not rig the grigri this way because if it is horizontal I would have to crank my wrist in a weird angle to keep the cam down as the rope feeds in. If you are having trouble visualizing I would try it out.

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