Page 57 - Colin Jarman "The Essential Knot Book"

15 SURGEON’S KNOT

This might be described as the Reef Knot for springy or slippery synthetics as it
performs the same task but is more secure in modern materials. There are two
forms; the more established is formed in the same way as a Reef Knot but with a
second turn at each stage. Thus the square knot becomes a rectangle as shown in
the diagram above left.

The second form (above right) has two initial overhand knots again, but instead
of a second pair on top there is just one. Then, when the knot is drawn up tight
the upper overhand knot forms a parallel crossover leaving the ends sticking out
on opposite sides. When drawn up tightly it forms a very secure, locked knot.
The photographs show the differences.