Disclaimer: I’d like to make sure it’s understood that these are in no way to be taken as qualified instruction. I suppose that technically I should classify these as ‘reminders’, but it is possible to learn from them… Just make sure you check with someone that actually knows what they’re doing before you try and do them halfway up a cliff.
The double fisherman’s is the knot to which all knots for joining ropes are to be judged against. Strong and bombproof. Standard use is for making prussiks and joining ropes for abseiling. Unfortunately, it jams massively (great for prussiks), making separating ropes after an abseil a pain, and it’s a very symmetric knot, meaning it could potentially get caught when you’re trying to retrieve it. Basically, just tie two stopper knots against each other. As always, leave long tails when joining ropes.
Lay the two ends next to each other, tails facing the opposite ways. Wrap one tail around the other rope.
Wrap it around again UNDERNEATH the first turn.
And once more, passing the tail up through the turns, AFTER the crossing point.
And pull tight.
Do the same with the other tail, and pull the lines apart, so the knots come together.
And the finished product: