A prussic knot is one that it tied with a loop of rope around a rope that is thicker than itself. typically is this means accessory cord (tied into a loop either using a double or single overhand knot or a fisherman's bend) commonly carried by climbers on their harnesses as part of a "rack".

It is tied around a climbing rope and allows the climber to ascend the rope, either by using another prussic loop, or by using a single prussic loop and a mechanical ascender, sometimes a belay devices is used in place of the mechanical ascender, if a belay device is used it will an "auto locking*" device.

  • auto locking belay devices may in some circumstances fail, for this reason it is important to have a fail safe, so that in the event the device fails, the climber does not hit the floor.

It is often used in the event where one climber is not able to complete a section of a climb, so the other climber proceeds to the top, and lowers the rope down so the second climber can ascend the rope and the two can carry on.

Step 1: The Way It Works.

Once complete and correctly tied the knot allows you to move it up and down the rope if you hold the knot itself to do this, but if you put load on the rest of the loop (the part that hangs down from the rope, it will lock solid, allowing you to put a foot in and stand in the loop, allowing you to move up. alternately it could be attached to your harness.
Is there any way you could reshoot the pictures with contrasting colored rope? I lost you in step 2 and never recovered.
 this is also known in the rigging world as a triple sliding hitch.  It is the only knot allowed to secure a worker above a 3m (10 ft) hight. 
Sorry about that. but I don't think I have anything that would really contrast with it, like bright yellow! Climbing rope being rather expensive! Colour wise with the access cord it was a matter of "colours vary, get what your given." I will how ever endeavour to find or make an illustration of what it should look like to make it easier to understand. I will also try and reword it to see if that makes any difference. I think once you have the basic Larks foot the rest is pretty self evident though because you actually repeat what you just did. In effect you tie a larks foot with in a larks foot.
It's worth noting for future users that prussiks are a complicated business. And more importantly, that they can and do slip. Avoid ever only being attached to a single prussik. There are many different prussiks knots which all work slightly differently and are better for different roles. Also the ratio of the diameter of the prussik cord and the rope it is being used on greatly affects the amount of friction generated. Thinner cord will bite more, fatter cord will slide more. Choosing the number of turns in a prussik requires experience to judge correctly. Getting it wrong can create a prussik which grips so hard that it is impossible to move, or one which is so loose that it fails to grip at all. Finally make sure the used to tie the two ends to create the loop for your prussik is sound. The author didn't cover that, and a well tied prussik won't help you if your loop opens.
So glad I found this site. I am a hunter and needed a way safetly up and down the trees that I hunt from, this is perfect than you.
Learned this knot after seeing Roger Moore ascend a rope with his shoelaces in a James Bond movie. Wondered, "how did he do that." Curosity can be powerful!
Could be done, if the laces were strong enough! Though they may be a bit short.

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