In reply to Jack Geldard - Editor - UKC: Good article Jack.
On the subject of cross-loading the figure-8 knot in the rope loop:
In a previous life I did some tensile tests for Steve Reid/Lyon Equipment. The context was for connecting 2 ropes for abseiling. We found that a cross loaded figure-8 knot was dangerous because it tended to invert repeatedly, in effect working it's way up the rope until it slipped off the end.
At the time, we both realised that
in theory this also could happen in the context of the rope tie-in loop, especially if you bring an anchoring rope back to the loop.
After an initial panic, we got sensible. After all, we don't hear about accidents involving rope tie-in loops coming undone. Looked at objectively, this is probably because:
1. Forces on the belay are generally low because of slippage through the belay device.
2. To pull the knot apart, you have to get the load configuration just right. Even then, often the rope simply doesn't invert. It seems to be worse if the rope is stiff and icy, but a lot of research would be needed to nail this down along with other factors such as rope construction etc.
3.
Using a snug stopper knot against the figure-8 knot prevents inversion. Most people are taught this as good practice, this is another good reason for tieing one .