Alpine Butterfly Loop

Also know as: Linesman's Loop

Uses:

This knot is used to form a fixed loop in the middle of a rope. Tied in the bight, it can be made in a rope without access to either of the ends; this is a distinct advantage when working with long climbing ropes. The butterfly loop is an excellent mid-line rigging knot, it is symmetrical and handles multi-directional loading well. In a climbing context it is also useful for traverse lines, some anchors, shortening rope slings, and for isolating damaged sections of rope.

Advantages:

Forms stable, secure loop after initial setting.
Allows for the knot to be loaded three ways; by each end of the main line and the loop.
Relatively easy to untie after loading.
It can also be used to isolate a worn section of rope, where the knot is tied such that the worn section is isolated in the loop.

Disadvantages:

Difficult to tie one-handed.
Difficult to tie around a solid ring or similar object.
Improper tying can result in similar looking but inferior "false butterfly" knot.