Knots/Loop knots/Alpine butterfly

The Butterfly Loop, also known as the Lineman's Loop or alpine butterfly loop, is a "non-jamming loop on the bight": a loop which may be tied in a rope with two fixed ends, and can take loads on both ends of the original rope, and on the loop. The alpine butterfly loop is a symmetrical and more secure version of the butterfly loop.

Usage
The Butterfly Loop has a high breaking strength and is regarded by mountaineers as one of the strongest knots to attach climbers to the middle of a rope, such that they have room to move around even when the main rope goes tight, and they can be supported in either direction from the main rope. The loop is typically attached to a climbing harness by carabiner.

It can also be used to isolate a worn section of rope, where the knot is tied such that the worn section is used for the centre of the loop (which of course does not receive a carabiner or bears any loads in this case)..

Advantages

 * Does not reduce the strength of the rope by very much
 * Will not slip (after initial settling)
 * Allows for the knot to be loaded three ways (each end of the main line and by the loop)
 * Relatively easy to undo after loading (more difficult if wet)
 * Very easy to adjust the size of the bight

Disadvantages

 * Difficult to tie one-handed
 * Difficult to casually inspect
 * Requires some training/practice to master