The Alpine Butterfly Loop, also known as lineman's loop, butterfly knot, alpine butterfly knot and lineman's rider, is a knot 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 handles multi-directional loading well and has a symmetrical shape that makes it easy to inspect.
In a climbing context it is also useful for traverse lines, some anchors, shortening rope slings, and for isolating damaged sections of rope.
Usage: 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 isolated in the loop (which of course does not receive a carabiner nor bear any loads in this case). The loop portion is isolated when the other two legs are loaded, and in fact the butterfly can be tied as a bend with the ends emerging where the loop would be.
Errors in tying the butterfly loop can produce a similar looking but inferior knot, the so-called "false butterfly", which is prone to slipping. However, some sources suggest this behavior can be exploited purposely for shock absorption.