Page 61 - Peter Randall "The Craft of the Knot.."

Also called the Alpine Butterfly, this loop is strong, secure, and easily
untied after being put under strain. It’s a great loop for use anywhere
along a rope, and holds well even with strain put on both standing parts.

STEP 1 Twist the rope twice to make two adjacent crossing turns.

STEP 2 Pull down the outer turn.

STEP 3 Bring the middle of the loop up through the center of the other
crossing turn.