Because the standard loop-to-loop connection is versatile, simple to use, and slides through rod guides easier than other knots, most of us rely on it at some point to fasten a loop on one part of the line system to a loop on another.
That connection can be between backing and fly line; fly line and leader; or within the leader configuration itself. The union formed by joining one loop to another is a strong one when properly done (see photo 1); however, if the loop-to-loop knot isn’t executed correctly by drawing the loops tightly together so that they remain seated in place, a hitch may develop at one of the loop ends and weaken the connection (see photo 2).
In some instances it is difficult if not impossible to keep the loops fitted tightly to one another. That situation occurs most frequently when utilizing some welded or hand-whipped loops at either end of a particularly stiff fly line (such as those that have monofilament cores and/or designed to be used in the tropics) and when attaching a loop made from leader material that is either thick or otherwise comparatively inflexible (i.e. butt leader to either tapered or big-game leader).
Furthermore, loops fashioned from a stiff fly line or leader material have lots of memory and tend to revert to their original rounded shape regardless of how much pressure is applied in trying to pull them taut so that they stay seated together. Under these circumstances, the loop connection tends to increasingly loosen over time until an unwanted hitch forms over one of the loop ends. The backing to fly line link is a frequent problem area, but a hitch can develop anywhere there is a loop-to-loop connection.
On the other hand, the problem is far less likely to occur with more flexible fly lines (such as those designed for cool waters) and leader materials where a single loop-to-loop connection is a better choice because it is more likely to stay in place after tightening.
When there is a high probability of a hitch developing in a loop-to-loop knot, I recommend using a double loop-to-loop connection instead – particularly when dealing with stiff fly lines or leader materials. To construct the knot properly, simply begin by making a single loop-to-loop connection. Then, instead of drawing on both loops so that they securely fit together, keep them separated and thread the tag end of one line a second time through the loop of the other line (see photo 3). Seat the loops by simultaneously drawing both lines in opposite directions as you would with a single loop connection (see photo 4).
It may take regular inspections of your loop-to-loop connections in order to identify prospective trouble spots, but once one is located, a double loop-to-loop connection will eliminate any future worries.