Surgeon's knot

The surgeon's knot is a reef knot with a reinforced first throw. In the ordinary reef knot the ends cross around each other once in the first throw; this single wrap has little friction and slips open as soon as you let go. In the surgeon's knot the ends are wrapped around each other twice. The extra wrap increases friction enough for the first throw to hold the tension on its own.

The second throw is then made single and in the opposite direction, just like in the reef knot. Opposite means: if you laid right over left the first time, you now lay left over right. This makes the two halves of the knot lie flat and symmetrical, and it stays closed.

In a correctly tied surgeon's knot you see a double wrap at the bottom and a single throw above it. The two ends come out on the same side and lie parallel to the cords they come from – just like in the reef knot. The knot lies flat.

If the ends stick out at an angle or the knot is twisted, you probably laid the second throw in the same direction as the first. That makes a granny knot, which comes undone easily. Pull the first throw snug before laying the second, then pull both ends evenly tight. Leave the tails long enough that they do not slip out.

The surgeon's knot is the most practical knot for tying something shut under tension: a parcel, a bin bag, a sack of supplies, a bundle of sticks, a rolled mat. It also helps with slick cord, which gives way at once with a plain reef knot. You can tie the second throw as a bow if you want to open the sack often – just like a shoe.

But it is not a bend for joining two ropes that carry load. Like the reef knot, it can capsize and come undone if an end and its own rope are pulled in an unfavourable way. For joining loaded ropes, use a sheet bend, a double sheet bend or a fisherman's knot.

The most common mistake is making the second throw in the same direction, which creates a granny knot. The second big mistake is using the knot to join two load-bearing ropes. If something heavy hangs from such a rope, the knot can suddenly come undone.

The name comes from surgery, where a similar knot form is used. But medical suturing is not the subject of this page, and wounds are not treated with improvised knots. For injuries: give first aid as well as you have learned it, and for serious injuries call the local emergency number (112 in the EU). Life-safety use of knots only after proper training with certified equipment.

Take a piece of string and a cardboard box or a rolled-up towel. First tie an ordinary reef knot and watch how the first throw springs open as soon as you let go. Then tie the surgeon's knot and let go after the first throw – the string stays tight.

Next, practise always making the second throw the opposite way. A saying helps: 'Right over left, then left over right.' Once you can tell a reef knot from a granny knot at a glance, you have mastered the surgeon's knot too.