A surgeon's knot is a square knot (right over left, left over right) but after the first time through the loop go through the same loop a second time, then complete the knot (right over left, again over left; left over right). The extra time through the first loop keeps the material (be it fabric or skin!) in place better until the second loop is finished, and also the whole knot is more secure. Try it in tying a quilt, and I bet you'll be convinced it's worth the tiny extra movement. I was!