The Yassa was the oral law code of the Mongols that served as the de facto legal framework for the Mongol Empire. It was gradually developed and built up during the reign of Genghis Khan, originating from wartime decrees that were later codified to include various cultural and lifestyle conventions.
While it functioned as the empire's primary law, the Yassa was kept secret and never made public, allowing for selective modification and use. It is believed that Genghis Khan and his adopted son Shigi Qutuqu, who served as the high judge of the Mongol Empire, supervised the code. Additionally, Genghis Khan appointed his second son, Chagatai, to oversee the execution of these laws.