A modification of a Reef knot with an extra twist at the bottom, used for joining 2 lines. It is not used in climbing or other situations that require high safety. This knot is common for tying suture threads (that’s where the name comes from). It is also applied in fly fishing.
ABoK #461, #463, #1209
Cross the running ends, putting the left one over the right one.
Pass both running ends around the standing parts downward, making an overhand knot.
Pass the running ends around the standing parts again, but upward.
Cross the running ends again, but placing the end of the right rope on top.
Pass the running ends through the loops to create an overhand knot.
Tighten the knot.
The Surgeon’s knot is complete.