What it connects

The loop-to-loop connection joins a loop in the end of the leader to a loop in the end of the fly line. Orvis's page says it is used to attach a loop leader to the permanent loop on the end of a fly line. Take Me Fishing calls it one of the best ways to attach a leader to the loop on the end of a fly line.

Nothing is tied between the two lines. Each side already carries a loop, and the connection is made by passing the loops through each other. That is why it is quick: Animated Knots, on its perfection loop page, says a loop-to-loop connection allows quick and convenient leader changes.

This page covers the connection only. For the loops themselves, see the two it names: the perfection loop (below) and the surgeon's loop in the double surgeon's guide.

Basis: consensus (Orvis, Take Me Fishing, Animated Knots)

Where the loops come from

Fly line: Take Me Fishing says fly lines now come with pre-made loops on their ends. Orvis's page says "permanent loop on the end of a fly line." Whether your line has one is on the line's package or in the maker's description, and we did not find a list of which lines do.
Leader: a loop is tied in the butt end of the leader. Take Me Fishing suggests starting with a perfection loop knot, and says a surgeon's end loop also works. Orvis's perfection loop video page could not be opened in this run (it refused the request), so we rely on Animated Knots and NetKnots for that knot. Some leaders are sold with a loop already in the butt; we found that only in retailer listings, not on a maker's page we could read, so we do not rely on it.
A fly line with no loop: makers sell braided loops to fit one. We read that in retailer listings, not on the makers' pages, so the details of how they are fitted are not covered here. Status pending.

The perfection loop, as Animated Knots gives it: form a loop in the end of the line, form a second loop and lay it on top of the first with the tag end under the standing end, pass the tag end between the two loops, pull the top loop through the bottom loop, lubricate, tighten by pulling on the standing end and the new loop, and trim. Animated Knots says it is the easiest way to make a small loop in the end of a leader or tippet, and that the loop lines up with the standing end. NetKnots describes the same knot as a favorite for a loop at the end of the line, one that can be made very small.

Basis: consensus (Take Me Fishing, Orvis, Animated Knots, NetKnots)

How to make the connection

Orvis, in both its video and animation pages, gives three steps:

Pass the loop on the fly line through the loop on the leader.
Pass the end of the leader through the loop on the end of the fly line.
Pull the line and the leader apart until the loops slide together. Orvis says the result looks like a square knot, and that you may have to help the knots pass through the loops with your fingers.

Take Me Fishing gives the same three moves in the same order and says to pull the fly line and leader in opposite directions until the loops slide together.

Orvis's page gives no wetting step and no tag or trim step, because nothing is tied. Wetting the loops is not something either page states for this connection, so we do not add it.

Basis: consensus (Orvis video and animation pages, Take Me Fishing); the two Orvis pages are one source

What it should look like, and the girth hitch

Orvis describes the finished connection as looking like a square knot. Cast and Spear, a fly-fishing site we treat as secondary, adds that it only resembles one: it is not a true reef knot, just similar in appearance.

The wrong result is a girth hitch, where one loop folds over onto the other instead of crossing it. The one source we read that names it is Epic Fly Rods, a rod maker's article: draw the connection tight so it closes as a square knot and not a girth hitch, because a girth hitch can weaken the connection. If you get one, Epic Fly Rods says to dress the knot by flipping the loop over, then pull it snug. Their article describes that fix for a loop of backing, and we did not find a source that says it works the same way on a leader loop, so treat that part as a thing to try, not a tested step.

None of the pages we read gives a drawn picture of the girth hitch, a figure for how much it weakens the connection, or a reason it forms. We have not drawn one either. Status pending.

Basis: official on Orvis's description; consensus on the girth hitch (one maker article); the application to a leader loop is not established

How to check it

The sources give one check, and it is visual: after you pull the line and the leader apart, the join should look like a square knot (Orvis, Take Me Fishing, Epic Fly Rods). Orvis states no other test.

Our additions, labelled as ours, not from a source:

Look at it with both loops snugged. If one loop appears to wrap the other like a hitch around a post rather than crossing it, do not fish it. Undo it and make it again, or try the flip Epic Fly Rods describes.
Scientific Anglers' general rule for every knot is to test it by pulling on it hard. That rule is for the knots it covers, which do not include this connection, so we cite it as a habit rather than as instructions for this join. Our own view: give it one firm pull before the first cast.
Look at it again after a fish or a snag. That habit is ours; no source we read says the shape changes under load.

Basis: official on the visual check; editorial on the additions, labelled in the sentences

Why anglers use it

Quick leader changes. Animated Knots, Take Me Fishing and Orvis all frame it as a way to attach a leader to a looped line, and Animated Knots names quick leader changes. That it comes apart by reversing the moves is our reading of the steps, not a sourced claim.
The loop on the leader can be tied before the trip, so only the interlock is left to do on the water. That is our reading of the steps, not a sourced claim.

Basis: official on Animated Knots' quick-change statement; editorial on the last two points

Conventional tackle

The brief for this guide asked about leader loops and pre-tied rigs on spinning and baitcasting tackle. We did not find a maker or agency page that gives a loop-to-loop connection for conventional tackle, and the pages above all describe fly line. We therefore say nothing about strength, line types or rigs for conventional tackle. Status pending. For the knots we do cover for conventional tackle, see the Palomar, the uni and the Alberto, and line systems for how the pieces fit.

Basis: Status pending

Limits

Strength is not published. Take Me Fishing calls loop-to-loop connections a strong and secure way to join two loops and gives no breaking figure; Epic Fly Rods gives none either. We found no test that measures the connection, and we do not put a percentage on it. Status pending.
It is a join between loops, not a way to attach to a fly. Fly attachment is a terminal knot; see the non-slip loop and the Palomar.
It needs a loop on both sides. If the leader has no loop, or the line has none and you have not fitted one, it cannot be made. The double surgeon's page covers the surgeon's loop; the uni and Alberto pages cover joins where a loop is not an option.
The loop knot sets the limit. A loop is only as sound as the knot that makes it. Animated Knots gives no line-type limit for the perfection loop and NetKnots gives none either; that is a gap, not a clearance for any line. See leaders and tippet for sizing.
Casting and guides. We did not read a source on how the join behaves through the guides or in the cast, so we make no claim about it.

Basis: consensus where cited; the rest is a gap in the record

What we don't know yet

A maker's own page (Orvis, RIO or Scientific Anglers) on how to check the connection beyond "looks like a square knot." The Scientific Anglers knot page we fetched does not cover loops. Status pending.
A published strength figure for the loop-to-loop connection, or for one that has formed a girth hitch. Status pending.
Whether the "flip the loop over" fix works on a leader loop as it does on the backing loop Epic Fly Rods describes. Status pending.
Whether the loop-to-loop is used on conventional tackle by any maker we could read, and how. Status pending.
Which fly lines ship with a loop, and how braided loop connectors are fitted. We saw these only in retailer listings. Status pending.
Orvis's perfection loop page, which refused our request. The loop steps here come from Animated Knots and NetKnots.
Wear on welded loops with use. We saw it mentioned in a third-party guide and have not read a maker saying it. Not used on the page. Status pending.

Sources and how confident we are

Claim	Basis	Confidence
Three-step connection	Orvis (video and animation pages); Take Me Fishing	High
Square-knot look as the check	Orvis; Take Me Fishing; Epic Fly Rods	Medium: no maker test
Girth hitch is the wrong result	One maker article	Low
Perfection loop steps	Animated Knots; NetKnots	High
Strength of the connection	Not published	Not available
Conventional-tackle use	Not found	Not available

Sources

Loop Loop Connection (video) — Orvis — used to attach a loop leader to the permanent loop on the end of a fly line; the three steps; looks like a square knot; help the knots through the loops with your fingers; no warnings or checks stated.
Loop Loop Connection (animation) — Orvis — the same text as the video page.
Loop to Loop Knot Connection — Take Me Fishing — one of the best ways to attach a leader to a fly line loop; fly lines now come with pre-made loops; the three moves; a perfection loop or a surgeon's end loop on the leader; a strong and secure way to join two loops, with no figures.
Perfection Loop Knot — Animated Knots by Grog — the loop steps; the easiest way to make a small loop in a leader or tippet; a loop-to-loop connection allows quick and convenient leader changes; no strength claim and no line-type limit.
Perfection Loop — NetKnots — a favorite for a loop at the end of the line; can be made very small; no line-type limit stated; does not mention loop-to-loop.
Connecting Loops in Fly Fishing Lines — Epic Fly Rods — close it as a square knot and not a girth hitch, which can weaken the connection; dress it by flipping the loop over; no strength figure.
Loop to Loop Knot — Cast and Spear — secondary: the connection resembles a square knot but is not a true reef knot. We did not use its strength claims, which cite no test.
Fly Fishing Knot Tying Basics — Scientific Anglers — fetched and checked; it does not cover loops. Its general rule to moisten knots, tighten slowly and test every knot by pulling hard is cited as a habit only.

Loop-to-loop