The cord on a katana, also called a sageo, is an invaluable component to a swordsman. During combat, it is used to secure a sword sheath to a swordsman's belt. However, when a sword is not in use, the sageo must be secured to the sheath. In Japanese tradition, this is done in a very ornate and complex manner. The end result is meant to be both functional and decorative.
Feed the sageo through the knob on the sheath. Place the sheath on a surface so that the mouth is to your left.
Pull the left strand over the knob and slide it under the sheath. Pull the right strand to the left until it is folded over the left strand.
Place your finger on the bottom side of the sheath between the left strand and the sheath to create a space. Fold the left strand into a loop and bring it over the sheath. Feed it through that space. Pinch the loop and pull to tighten it. Repeat two times, creating a total of three loops to the right of the knob. Add each subsequent loop to the right of the one before it.
Take the right strand and slide it under the sheath. Repeat the same process you did with the left strand, this time creating three loops to the left of the knob.
Fold the remainder of the left strand in half and slide it -- bent end first -- into the seven loops. Do the same with the right strand
If you have trouble sliding the remainder of the sageo into the loops, try using a long object, such as a pen, to widen the loops.
Katanas are very sharp weapons. Always use caution when handling them.