The Sai was a weapon developed to defend against the sword, staff and empty-hand attacks. In the beginnings of the martial arts, harsh penalties were often given to anyone possessing weapons. Many farm tools were therefore developed into defensive tools. The Sai was believed to have originated from the pitchfork, and is one of the five systemized weapons developed by the practitioners of Okinawan karate.

For the black belt, free sparing as well as a thorough knowledge of weapons is mandatory. Some styles insist on various weapons Kata for rank advancement especially at the Black Belt level. Many styles judge a Black Belt applicant on his character, spirit, confidence and to fully control all his movements in kata and Kumite.