The diamond knot is a knot for forming a decorative loop on the end of a cord such as on a lanyard

Lanyard

A lanyard is a rope or cord exclusively worn around the neck or wrist to carry something. Usually it is used where there is a risk of losing the object or to ensure it is visible at all times. Aboard a ship, it may refer to a piece of rigging used to secure objects...

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Tying

The diamond knot begins as a carrick bend

Carrick bend

The Carrick bend is a knot used for joining two lines. It is particularly appropriate for very heavy rope or cable that is too large and stiff to easily be formed into other common bends. It will not jam even after carrying a significant load or being soaked with water...

with the ends exiting diagonally opposite each other. When the steps below are completed the knot is rearranged and tightened so that the ends emerge from the knot parallel and opposite their own standing part.