Visitors to the Forbidden City in China’s capital city might find it hard to ignore its magnificent red door, but there is more to the entrance than its beauty.

The golden door nails not only neatly line up, they were exclusive to the royal family and cannot be seen everywhere in China.

In the Ming (1368–1644) and Qing (1644–1912) dynasties, door nails were a symbol of hierarchy, and are bound by strict rules.

Firstly, the number of door nails must be odd because in traditional Chinese culture, odd numbers stand for yang, or the light, and even numbers stand for yin, or darkness.

Nine is the biggest of odd number under 10, so it became exclusive to the emperor, symbolising the supreme power and divine authority.