Page 24 - Patrick Moreau - Marine Knots How to Tie 40 Essential Knots

The oysterman’s stopper knot is useful when a stopper larger than what is
provided by the simple knots is needed.
Its triangular base acts as an excellent stopper.

1 and 2 - By once again using the same starting movement, first lead the working end twice over and then
under to create an “A” bight twice-under, which you can pull out the top.

3 - The working end then goes all the way around passing over the standing end to cross the bight that it just
came out of. The closure will only be correct if you are careful to adjust each bight one at a time. At the
end, you should see the base triangle.