Reef Knot

The Reef Knot is basically a Standard Shoelace Knot minus the loops. This is useful when the ends are not long enough to tie a regular bow, such as when a shoelace has broken. Also known as a “Square Knot”.
Step 1:

Begin with a regular Left-over-Right Starting Knot.
Note the short ends in this diagram, which are an example of where the Reef Knot is probably most needed.
Step 2:

The steps for tying are the exact opposite of the Starting Knot. Cross the right (blue) end over the left (yellow) end. The right (blue) end is now on the left side.
Step 3:

Begin to wrap the left (blue) end around the front of the right (yellow) lace to end up at the back of the gap between the laces.
Step 4:

Feed the left (blue) end through the gap to emerge at the front left-hand side. Pull tight to complete the knot.
Untying the Reef Knot
Most shoelace knots are untied by simply pulling on either or both of the ends. These act as “drawstrings”, which pull the loops back through the knot until it falls apart.
Because the Reef Knot has neither loops nor drawstrings, many people resort to picking it apart. However, it can also be untied as follows:
Untying – Step 1:

Pull either end straight upwards until its curves have been pulled into a straight line. As seen here, the right (yellow) end has been pulled straight. The left (blue) end is still wrapped around the right (yellow) end with a double-loop.
Untying – Step 2:

Simply slide the left (blue) double-loop up and outwards along the straight (yellow) lace until it falls off the end.
Finished Knot

The finished Reef Knot is identical to the centre of a Standard Shoelace Knot minus the loops.
Note the two very short ends, which would be nearly impossible to form into a regular shoelace bow.
Technical Details
Military Usage
I've been told that the Reef Knot has been used by those in the U.S. Army, with the ends simply tucked into the top of the boots – the theory being that there are then no loops that could snag and come undone.
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