What is the meaning of the door pin and auxiliary head on the gate of the Forbidden City

Door nails have several roles, one is decorative, one is to represent the rank, and then a reinforcing role. The earliest door nails only play the role of reinforcing the door plate. As a large door is often put together by a number of panels, which tend to fall apart over time. In order to avoid falling apart, in the door plate on the head of the band, but I am afraid that the band is not strong, and then reinforced with door nails. Later, the door nail made more and more neat, horizontal and vertical lines, the number of nails also became a sign of rank. The Forbidden City noon, Shenwumen, Xihuamen three doors are set up nine road door nails, that is, nine rows and nine columns, *** eighty-one door nails, the only East China Gate is eight rows and nine columns, *** seventy-two door nails. Auxiliary head is installed on the door on the door ring of a base, is the traditional Chinese door decoration. Imperial palace door on the auxiliary head, copper gilt, the image of the tiger, chi dragons, turtles, snakes. Symbol is the stars and lodges guard the door, can foretell the bad luck, but also useful for staring at the mouth of the lion as the auxiliary head of both the guardian of the meaning of the door, but also shows the greatness of the royal building and the solemn.