According to the theory of yin-yang and five elements, the north belongs to water. Its gate is called "Tianyi", which means "Yi Tian and Shui Sheng" in the Book of Changes and corresponds to the Five Elements Theory. Because the Forbidden City was burned many times during the Jiajing period, the walls and gates of Qin 'an Hall were built under this background. The gates were inscribed by Emperor Jiajing and also meant to pray for peace.