Double Shoelace Knot
Also known as the "Shoe Clerk's Knot", this is a crude method of making a knot "permanent" or for consuming excess lace to keep the ends from dragging: Tie a Standard Shoelace Knot, then tie an overhand knot with the finished loops. Unfortunately, it's also very tricky to un-tie (though that's great for preventing infants removing their shoes!)
Cross the left (blue) loop over the front of the right (yellow) loop to end up on the right side.
Begin to wrap the right (blue) loop around the back of the left (yellow) loop so that, like the Two Loop Shoelace Knot, the end can be fed into the gap between the laces.
Having formed another overhand knot, continue pulling on both loops until tight.
The finished knot is quite bulky, which is helpful for consuming excess shoelace to keep the ends from dragging.
NOTE: Many people resort to this crude method to stop their shoelaces from coming undone, which is often because they are inadvertently tying a Granny Knot. Resolving that problem will usually eliminate the need to tie such a cumbersome knot.
This knot is shown in the definitive Ashley Book of Knots as #1215, "The Shoe Clerk's Knot", where it is described as a "Bowknot with the addition of an opposing Half Knot tied in the two loops."
Untying the Double Knot
Unlike most other shoelace knots, the Double Shoelace Knot cannot be untied by pulling on the loose ends - it just won't budge! Most people resort to picking apart the final knot so that it reverts to a regular bow, which can then be untied in the regular manner.
However, if the final knot has been tied fairly loosely, there's a much quicker way!
The Double Shoelace Knot can usually be untied in one motion by pulling very firmly on only one of the lace ends (as opposed to pulling on both ends). If this fails, try pulling on the other lace end instead. If this also fails, picking the knot apart is then the only option. (This will be more difficult now that the knot has been pulled even tighter!)
Variation for Easier Untying
As suggested by Brian S., when doing the final tightening at Step (4), grasp one loop by the end of the loop while grasping the other loop by one side only. When tightened, this will leave a small loop sticking out the other side (the part that wasn't being held and tightened). When the time comes to untie the Double Knot, pull on this loop and the knot will easily come apart.
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