Reef knot

The reef knot consists of two half knots tied in opposite directions. This makes two bights interlock like two hooked fingers. Each end leaves the knot on the same side as its own standing part. The result is a flat, symmetrical knot that sits well against a bundle.

What holds it is mainly what it encloses. The bundle presses against the knot from inside, the two bights press against each other and cannot slide. If that counter-pressure is missing – for example when two ropes are joined with it and pulled apart – the knot loses its stability.

If one end and the standing part beside it are pulled apart, the reef knot capsizes: it flips into two lark's heads that slide freely along the rope. Sailors use this on purpose to release it quickly. With two loaded ropes, exactly this capsizing is dangerous.

A correct reef knot lies flat and symmetrical. Each short end lies parallel beside its own standing part, so both ends come out on the same side of the knot as their own strands. From above you see two neat, interlocking bights.

If instead the ends stick out at an angle and the knot looks twisted and lumpy, you have tied a granny knot: both half knots went the same way. The granny slips or jams and is unreliable even as a binding knot – retie it. The memory aid is: 'right over left, then left over right.'

To set it, pull evenly on all four strands until the knot sits firmly against the bundle. Leave the ends long enough to grip easily.

The most important rule for this knot: never use it to join two ropes that will be loaded. It can capsize – especially with ropes of different thickness or stiffness, or when one end is jerked – and then come completely apart. Standard knot references warn explicitly against this use.

The sheet bend is meant for joining two ropes and works even with different diameters. In slick synthetic cord under load, the reef knot is unreliable too. 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.

As a binding knot the reef knot is excellent: for tying up a parcel, a bundle of firewood or poles, a rolled-up tarp or sleeping mat. It lies flat, doesn't press and can be released on purpose by capsizing it. As a bow with two bights (like on a shoe) it comes undone with one pull on an end.

It is also traditionally taught in first aid for tying a triangular bandage, because it lies flat. But there the rule is: what counts for first aid is a proper course, not a knot book.

Tie a cord around a book. Make the first half knot right over left, the second left over right, and compare with the picture: do the ends lie beside their own strands? Then deliberately tie a granny knot next to it and compare the two. Finally: pull one end and the neighbouring standing part apart and watch the knot capsize – then you will understand the warning by yourself.

The knot takes its English name from reefing sails: reef points were tied with it around the gathered sailcloth. There the cloth presses against the knot from inside – exactly the situation it is made for. In American English it is also called the 'square knot'.