A butterfly (also known as an alpine) is used to put a bight of rope along the length of rope. Under tension, the bight maintains its shape. The knot is a series of two loops holding a bight in place. The advantage of the butterfly is that it can be tied mid-line, is easy to tie and identify, and the bight can be made to be as large or as small as desired.
Hauling equipment and materials, litter attendants and multiple attachments.
Form a bight of rope and wrap the rope two times around your hand. Then wrap your hand a third time, wrapping over the second wrap. Pass the top strand (towards your fingers) down the palm of your hand. Insert the working strand between the palm of your hand and the strands wrapped around your hand. Continue pulling on the working strand over the top of your fingers. Pull the knot off your hand and pull the two standing ends so they are linear.