Variant Names: butterfly knot, lineman's loop(Ashley).
Category: loop on the bight.
Origin: Ancient.
Related knots:
Releasing: Non-jamming.
Efficiency: Unknown%.Compared to other knots that might be used for the same purpose it is stronger (does not reduce the strength of the rope by as much).
Caveat: None, secure if wet.
Uses: climbers, mountaineers in order to tie on in the middle of a rope.
Comments:
Structure:
Tying: article/pictures
http://www.geocities.com/roo_two/butterflyloop.html