The alpine butterfly is a useful knot as it enables a loop to be tied in the middle of the rope. While you could use a figure-of-eight loop, it has the ability to unroll, and is less strong in this situation.
|Pass the length of rope across your palm.|
|Wrap the rope around your hand to make a complete turn.|
|And once more.|
|Reach under the outermost loop with your other hand and pick up the middle loop|
|Pull the middle loop out and over the other two loops.|
|And then feed it back through the other two loops.|
|Pull the three parts of the knot (the two ends plus the loop) to tighten and set.|