The loop-to-loop is the simplest connection there is: two lines, each ending in a loop, interlocked so they hold. It is not fancy, but it is one of the handiest tricks in fishing, because it lets you swap leaders and pre-tied rigs on and off in seconds without tying a single knot on the water.
When and why to use it
Use it to connect a fly line loop to a leader loop (most modern fly lines come with a welded loop), to swap between pre-built leaders, or to add a pre-tied rig or shock tippet quickly. Build the loops at home with a nail knot loop or a perfection/surgeon’s loop, and on the water you just connect them, huge time-saver.
How it works
Pass one loop through the other, then pass the far end (or the whole leader) back through the first loop and pull them snug so the two loops interlock in a square, luggage-tag connection. Done right they seat into a neat, strong link; done wrong (a “girth hitch” where one loop cinches across the other) they can be weak, so make sure they interlock squarely.
Tip Rig several leaders at home with loops, and carry them ready to go. When conditions change, or a leader gets chewed up, a loop-to-loop lets you swap to a fresh one in seconds instead of standing there re-tying, which matters most when the fish are up and biting.