Yes I know that there are plenty of other cool knots out there ... many of which I literally couldn't live w/o ... I rock climb. However, this knot is unlike any other. Plus I needed a grabber for my Instructable. :-) (BTW: this ISN'T a climbing knot!) And in truth, it isn't a knot by itself, but rather a system of common knots.
Have you ever tried to tie something down for transporting, but just couldn't get the lines tight and/or during transport the lines would continually loosen? Then this is the knot for you! I learned this knot back in the 70s when specialty car racks and ratchet straps were rare or unheard of. I initially used it to tie a canoe on a car rack, both attaching to the rack as well as the lines to the bow and stern of the canoe. Even with all the new gizmos available today, this knot still shines because all you need is a rope and ropes don't hum in the wind like straps.
The unique aspect of this knot is that it gives you a 2-1 mechanical advantage when tightening the rope. Be careful though. You can actually damage some things because of the mechanical advantage. This knot holds fast and is easy to untie, hallmark traits of any good knot.
Below you see the finished knot system ... we'll break it apart in the steps that follow
Step 1: Initial setup
The first step is to anchor one end of the rope and then loop the rope around a 2nd anchor point.
For attaching to the 1st anchor point I chose bowline ... a close #2 on my list :-). There are other instructables on that one so I won't bore you here.
The 2nd anchor really should be round because it serves as a pulley in this block and tackle type knot. I've used it on sharp anchor points and it doesn't work as well.