The cinch knot is a flat, adjustable knot (made famous by cowboys). It was, and still is, used to hold a saddle in place on a horse's back. Though newer saddles now use a buckle, the cinch knot (also called a girth hitch) has been used for this purpose as long as men have been putting saddles on horses. The strap that goes under the horse is called a cinch strap, hence the name. When secured to a single ring under the stirrup it is very functional, secure, out of sight, and because it is flat it won't rub a sore on the horse.
This handy knot is also used by fishermen in the construction of hand made nets. It is the knot that secures the net to its framing or the bolt rope for hauling. This knot could also be used to create a tie point on a tarp being used as a survival shelter.
From under the rail, come out and cross over the standing part.
Go up behind the rail and forward over the top.
Tuck the bitter end back down through the existing loop alongside the standing part.
Snug it up and you are done.
How To: Tie a cinch knot in the middle of a line
Form a bight where you want the knot.
Grasp the 2 standing parts a few inches or so below the end of the bight.
Open the bight into an eye while still holding the 2 standing parts with your other hand.
Fold the eye over so it is bisected by the 2 standing parts.
Wrap both halves of the eye around the standing parts.
The resultant loops can be secured to the stub of a branch, a hook, or the end of a pipe and drawn up snug into a proper knot.
Riggers also use it and folks who do fancy work with any size line. When used in the middle of a line you can put a load on either end or both and it will not slip or bind. Even when tied in stretchy line like nylon, it is still easy to untie after it has been under a load. The cinch knot can be tied in the middle of a line or in the end depending on your need.