The big advantage of the figure-eight knot is that it is very easy to untie. That's why you should not use a simple key instead (undoable after a strong pull).
The figure-of-eight knot is used to complete a sheet, halyard, reefing line, etc. With it, the manoeuvre (the rope) cannot get out of the block, sheave, blocker or jammer.
We cross the strands over.
Before crossing underneath again.
Then to enter the loop formed.
Before tightening, the knot forms a beautiful "8" (hence its name).
