The alpine butterfly loop is a knot used to form a loop in the middle of a rope. Once tied, the loop will not shift up and down the rope. It is good for adding a stationary loop in a continuous piece of rope, without bending the rope.
Among high quality knots, the butterfly loop is perhaps the easiest to remember how to tie correctly. Start by simply making two twists in the same direction to form the two loops. Then wrap the outer loop around the standing part and pull it through the hole of the inner loop.