The Yassa was the oral law code of the Mongols, gradually built up through the reign of Genghis Khan. It was the de facto law of the Mongol Empire, even though the "law" was kept secret and never made public. The Yassa seems to have its origin in wartime decrees, which were later codified and expanded to include cultural and lifestyle conventions. By keeping the Yassa secret, the decrees could be modified and used selectively. It is believed that the Yassa was supervised by Genghis Khan himself and his adopted son Shigi Qutuqu, then the high judge of the Mongol Empire. Genghis Khan appointed his second son, Chagatai, to oversee the laws' execution.