1. It acts as a seal;
2. It prevents the deceased from being disturbed.
Generally, the "encoffinment" ranges from the day after death to the second day and the third day. It is usually three days for the encoffin to be completed. At this time, if the deceased is a woman, her maternal family must be present, especially if the deceased is not allowed to be buried without the approval of the uncle.
After the body is put into the coffin, the lid of the coffin must be nailed. Seven nails are used to hold down the nails, commonly known as "children's nails", which are said to make children and grandchildren prosperous. When the nails are being hammered, the relatives all shout to the deceased to "hide the nails" and then sprinkle five-colored grains on the coffin lid to complete the burial. After that, paper was burned three times a day to offer sacrifices, which was called "twisting incense", until the funeral.
In ancient times, coffins did not use nails, and leather strips were used to bind the bottom and lid of the coffin together. Bind three lines horizontally and two lines vertically. The wooden planks are long in the horizontal direction and short in the vertical direction. This is where the phrase "three long and two short" comes from.
Older coffin-making craftsmen do not use nails. It is said that the usage and position of nails are quite particular, which coincides with some extended meanings.
In fact, the purpose of nailing the coffin lid is to seal it. In the past, many people were not buried quickly after death. They had to leave the coffin for a long time. Nailing the coffin and sealing it could prevent the emission of various harmful gases after the body decomposed.
Another theory is that the nails are hammered into the coffin so that the deceased will no longer be disturbed, allowing him to enter the underworld smoothly. Therefore, the bottom of the coffin is very thin. People believe that this will facilitate the deceased to enter the underworld and be reincarnated smoothly.