Also known as the Double Loop, Surgeon's End Loop, or Surgeon's Loop Knot.
The Surgeon’s Loop is the easiest way to create a terminating loop at the end of a line. Simply create a double line at the end of a line and then start an overhand knot. Before closing the knot, run the double line through the opening again to make it a double overhand knot, then pull the end of the leader taut to complete the surgeon’s loop. This loop knot can be used to quickly swap in terminal tackle with large eyes, such as clips, snaps, swivels, or sinkers. It can also be used with larger hooks, provided that the loop can fit through the hook eye.