The jo is a short stick, approximately 40 - 50cm in length.

Due to the history between Japan and China, where people were not allowed to carry any weapons, the people were forced to find other methods and instruments to protect themselves with. One of the weapons they used was the jo. Since it was only a short stick, it wasn't considered to be a weapon by the authorities of that time, and could therefor be carried on one's person. It was also readily available along the roadside.
Hundreds of techniques were thus developed, ranging from strikes to locks to takedowns to strangleholds.
Using the correct methods this little stick could become quite lethal.