Reef knot

Binding knot · easy · also called Square knot

Diagram of a reef (square) knot, showing each end beside its own standing part. CountingPine, Public domain, via Wikimedia Commons

Photo of reef knots: one binding a loop of line (top) and one showing the knot's structure in two colours (bottom). USCG PTC Developer, CC BY-SA 4.0, via Wikimedia Commons

Used for

Tying reef points or sail ties around a reefed or furled sail
Finishing the ties on a sail cover, awning or canvas lacing
Tying off the ends of a whipping or other small stuff

How to tie it

Take one end in each hand around the bundle you are tying.
Cross the right-hand end over the left and tuck it under, like the first half of tying a shoelace.
Cross the ends again, this time left over right, and tuck it under.
Pull all four parts to draw the knot tight.

Check it: Each end lies alongside its own standing part, both ends come out on the same side, and the knot looks like two interlocking loops. If the ends stick out crosswise, it is a granny knot, which will not hold; retie it, reversing the second tuck.

Cautions

Never use it to join two lines that will carry a load, such as tow lines, dock lines or a lengthened sheet. It can spill (capsize) and come apart, and knot authorities blame it for many accidents. Use a sheet bend.
It will not hold in lines of different sizes, in wet line, or in slippery synthetic line.
Under heavy strain it can jam hard. Where a binding matters, the Army finishes it with an overhand knot on each side.

To untie: Push the ends toward the knot, or pull one end back hard along its own standing part, and the knot collapses.

Sources (7)

media.defense.gov · higherlogicdownload.s3.amazonaws.com · globalsecurity.org · marines.mil · boatingcompass.org · en.wikipedia.org · navytribe.com

General guidance for recreational boaters, researched from the sources listed and written in our own words. Your equipment manuals, local knowledge, the Navigation Rules and a qualified instructor come first.