Sheet bend

The thicker (or stiffer) rope forms a bight. The thinner rope comes up through the bight from underneath, goes around behind both legs and is then tucked under its own standing part – not through the bight. The thin end now lies across the bight, nipped by itself.

Under load the bight presses against this nip: the harder both ropes are pulled, the tighter it sits. Why the thick rope should form the bight: a stiff, thick rope keeps its U shape, while the thinner, more flexible rope can wrap snugly around it and grip firmly. The other way round, the thin U would deform under the thick rope.

Interestingly, the sheet bend follows the same rope path as the bowline. In the bowline, the 'bight' is simply its own large loop. Anyone who knows one quickly understands the other – and also knows that both share the same weakness: they can loosen when unloaded.

Look at the two short ends. In the form usually recommended, both lie on the same side of the knot. If they lie on opposite sides, you have a variant regarded as less secure – retie it. Also check that the thin end is nipped under its own standing part and doesn't simply run back through the bight.

Set the knot by pulling on both standing parts. Afterwards the tails should still project several rope diameters. With a large difference in thickness or slick rope, use the double sheet bend: the thin end goes around the bight twice before being tucked under itself.

Common mistakes: using the thinner instead of the thicker rope for the bight; tucking the end through the bight instead of under its own standing part; leaving the tails too short. Each of these can let the knot slip under load.

The typical failure mode is loosening: if the join is unloaded or flogged, the thin end can wander out bit by bit until the knot comes undone. With a very large difference in diameter, the single sheet bend is also less reliable. The remedies are the double sheet bend, long tails and regular checks.

The sheet bend is the right choice where the reef knot would fail: extending a line with a second rope, especially when the two differ in thickness. It also works for attaching a line to a corner eyelet or loop – the loop then forms the bight. It is quick to tie and usually easy to untie after loading.

It is not suitable for very slick or stiff ropes without the doubled form, for unloaded, flapping joins that nobody checks, or for lifting loads. None of the knots in this atlas is meant for life-safety use – not for climbing, abseiling, rescue or holding and lifting people. Life-safety use only after proper training with certified equipment.

Take two cords of different thickness, for example a rope and a piece of paracord. Form a bight with the thick rope and hold it in your left hand. Lead the thin end up through it, around behind both legs and under itself. Pull tight and check that both ends lie on the same side. Then practise the double sheet bend and compare how much firmer it feels.

The name comes from the sheet, the line used to control a sail – it was attached to the sail's corner with this knot. Because weavers are said to have joined broken threads with a form of this knot, it is also called the weaver's knot.