We climbed on a single rope, trad style with leader and second finishing at the top. Descent off this route requires an abseil. I used a sling with two carabiners as a cow's tail between my harness and the chain while we untied from the rope to set up the abseil. I clipped the cow's tail to my knot loop instead of the belay loop on my harness. I did not realise until some time after I had untied the rope - until then I had not been connected to the chain by anything, but had believed I was securely attached.