Prusik knot is a kind of double lark's head
Make a loop of line using an over hand knot. Attach the loop to a
straight line with lark's head knot, and continue making one or two
(recommended) more twists around instead of one. Make tight and tidy.
The knot should still slide on the straight line. Bend the Prusik knot
to U-shaped and it locks.
Prusik knot and its variants are used by mountain
climbers; if they hang their lives on Prusik knot, you can do it to
Making Prusik knot