The square knot extension is the easiest and most common way to extend your rope when tying a complicated harness.
It is secure enough to be used in the middle of a harness but is completely unsuitable for tying in any free-standing line. It will almost certainly fail if used to extend a vertical line.
1We’re going to extend the green rope by adding the blue rope to it.
2Make a lark’s head in the blue rope.
There’s a clever way to do this one-handed by folding the bight over the standing part.
3Pass the green rope through the lark’s head and back in the direction it came from.
4Flip the loop of the lark’s head over so that it lies on top of both parts of the green rope.
5Snug the knot into place by pulling simultaneously on the blue rope and both parts of the green rope.
6The finished knot should be small, tight, and secure.
This knot doesn’t appear in ABOK.
This is just a clever way to make a Square Knot (ABOK 1402) tied in one doubled rope and one single rope.