Nails on the doors, this tradition has long existed since the Sui and Tang dynasties (581-907). Specifically, in the Forbidden City, all four main gates have nails, but besides this, only the Meridian Gate has five gates, while the other gates have only three gates.
Except for the East Hua Gate, which has 8 rows horizontally and 9 rows vertically of yellow nails (8*9=72) with an even number (Yang number, which is an integer divisible by 2 and multiple of 3), all other gates have 9 rows horizontally and 9 rows vertically, meaning 9*9=81, which is an odd number (Yin number). The Chinese often use Yang numbers, while the Vietnamese tend to prefer odd numbers (Yin numbers). Through the use of nails on the doors, besides their structural and decorative functions, it is also seen to reflect the solemnity and grandeur of the feudal regime of that time.
Door frame
There are many questions about the choice of Yang numbers on the eastern gate (or the East Flower Gate). This is a mystery that historians have not yet found an answer to. Some believe that when Emperor Jiaqing and Emperor Daoguang were buried, they had to borrow this gate to bring the coffin out. Therefore, this gate is often called the Ghost Gate. This explanation may be reasonable because with Yin numbers, the deceased return to the Underworld. The number of nails fixed on the gates was determined according to the rank and class of the owner in the feudal system.
Since the Son of Heaven is the son of the Heavenly Emperor (Heaven), the number of rooms in the Forbidden City must be less than the number of houses that Heaven has in the celestial palace (10,000). This number is a Yang number and represents infinity in China. According to a survey in 1973, there are 8,704 rooms in the Forbidden City (a Yang number).
As for the color yellow, in Yin-Yang and the Five Elements, this color is usually associated with the Earth element and is located at the center in the management of all things and supervision of the four directions. Because it is the color of the midday sun, this brilliant color belongs only to the emperor as it symbolizes the respect and protection of the Heavenly Emperor. There was a custom of not using certain colors during the feudal period: red, yellow, and sky blue.
Incense Burner
Conversely, the common people used: black, white, and gray colors. Therefore, it is not surprising to see the prominent use of these two standout colors: purple-red and yellow in the construction of the royal palace in the Forbidden City. The walls are purple-red, while the glazed tiles of the palace are yellow. However, there are exceptions, all related to the theory of Yin-Yang and the Five Elements. That is the Wenyuan Palace, where the royal library is located. This place has a black roof. Fire was a concern in the Forbidden City. Fires were discovered many times in the Forbidden City. The last fire occurred when Emperor Guangxu was about to marry his cousin Longyu in one month. This was an ill omen for the marriage.
Using this as a pretext, Empress Dowager Cixi immediately executed the two eunuchs responsible for arranging the lanterns. Therefore, the color black, symbolizing water, was used in the Wenyuan Palace to prevent fire disasters and to protect the books in the library. Some houses near the East Flower Gate have roofs painted blue because this is where the princes lived. This is also the blue color assigned to the east in the Five Elements theory.