Page 24 - Colin Jarman "The Essential Knot Book"

01 REEF KNOT

Originally used to tie off the ends of reef points when shortening sail, this
symmetrical, flat knot can be formed with load on both standing parts and can
also be untied under tension by pulling on one end and capsizing the knot. It is
unsuitable for joining ropes together, but if used to do so, the ropes must be of
equal size and type otherwise it will slip.

A Reef Knot is formed with two overhand knots tied in opposite directions so
that ends lie back alongside standing parts on their same sides.