The Fisherman's knot is used to tie two ropes of equal thickness together. It is used by fishermen to join fishing line, and is very effective with small diameter strings and twines.
Tie a Thumb knot, in the running end of the first rope around the second rope. Then tie a thumb knot in the second rope, around the first rope. Note the Thumb knots are tied such they lie snugly against each other when the standing ends are pulled.
When tying knots in monofilament line, moisten the line before pulling the knot tight. This helps to stop the line heating up with friction, which weakens it.