Bob Wightman

The Double Fisherman's knot

Despite the uptake of knots like the overhand for tying two ropes together, the double fisherman's remains the safest way to do this in the climbing environment. It is also the only knot to use to join ropes of different diameters as it is resistant to the unequal stretching in the ropes that this can induce.

Attribute Notes
Strength 70%
Ease of Tying Not simple - it is easy to get the orientation of the loops wrong.
Recognisable Quite easy.
Also known as Grapevine Knot
Usage Tying two ropes together end to end. Often used to tie the cord on larger chocks in this fashion.
Advantages Very strong.
Can be used to join two ropes of different diameters.
Disadvantages Getting undone after loading!