The fisherman's knot (also known as the angler's knot, the Englishman's bend, the English knot, true lover's knot, and Halibut knot/bend) is a bend made for joining two ropes of the same size. It is most useful when the two ropes are stiff and/or small (such as fishing line - hence the name). Once pulled tight, it is very difficult to un-tie.
The knot consists of two interlocking overhand knots (aka half hitches), and is very simple to tie. If not pulled tight, the two individual knots easily slide along the rope they are attached to, however once tension is applied, each knot stops the other from sliding further and simply continues to tighten. For this reason, this bend is in a group of knots referred to as double-stoppers.
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Thanks again to tkil for his node on drawing knots with ascii art. It's not as easy as it looks.