What are the copper rivets on the ancient city doors for?

The doornail has several functions, one is reinforcement, the other is decoration, and the third is representing grade.

The earliest doornail only reinforced the door panel. Because a door is often made up of several boards, it is easy to spread out over time. In order to avoid scattering, I put a belt on the door panel, so I reinforced it with a doornail for fear that the belt was not strong. However, because the nail cap exposed on the surface of the door is unsightly, for the sake of beauty, people make the nail cap into the shape of a bubble head, so it has a decorative effect.

Before the Ming Dynasty, the number of doornails used was not specified. It was not until the Qing Dynasty that the number of doornails used was graded. "Qinghui Hall" records: "The doors of the palace are all worshipped, covered with yellow glass, and the doors are inlaid with gold nails." For the specification of "four doors on the inner and outer walls of the altar and temple, all with gold nails and nine doors vertically and horizontally", the number of door nails decreases in an odd number according to the following grades. Generally, the doors of the prince's residence are nailed horizontally and vertically; The doornail of the prince's residence is vertical and horizontal; The duke's doornails are seven in vertical and horizontal directions, and the marquis and baron are five in vertical and horizontal directions. But their respective doors can only be nailed with iron gates, not with copper doornails. Tiananmen is the main entrance of the imperial city, and of course the doornail is also the highest level.