Alpine butterfly knot
also known as lineman's loop, butterfly knot or lineman's rider.
this knot is used to form a fixed loop in the middle of a rope.
it’s 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.