Yassa (alternatively: Yasa, Yasaq, Jazag, Zasag, Mongolian: Их засаг, Ikh Zasag) is/was the oral law code of the Mongols declared in public in Bokhara by Genghis Khan 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 as 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 stepbrother Shikhikhutag, then the high judge (in Mongolian: улсын их заргач) of the Mongol Empire. Genghis Khan appointed his second son, Chagatai (later Chagatai Khan), to oversee the laws' execution.