Lapp
From the arctic region of Lappland, this is quite possibly the simplest way to join two ropes together. It can be done even if your fingers are frozen from the cold.
Make a fold in the end of one rope. You are now totally done with everything you need to do with that end. Simple? Wasn’t it?
With the other end, make a loop.
Drop the loop over the fold.
Memo: if you try making this knot and it comes apart, then you put the loop on upside down. Try again.
Take the end of the rope on the side that has the loop (remember, you are already done with the folded side so don’t take the end from there) and pass the end through the fold.
The completed knot is very secure, and relatively strong. The knot-geek who is ‘in the know’ might recognize this as a classic scouting sheet-bend turned sideways.