Because in nature, human and animal bodies are symmetrical, and all organs exist in pairs, hands, feet and eyes; Leaves and flowers of plants also grow symmetrically and exist in pairs or even numbers. The veins of leaves are symmetrical, and so are the petals. So the ancients thought even numbers were auspicious and a symbol of life. Many things are noticed in pairs to show good luck. For example, the number of gifts given on holidays is generally two, four or six, and two bottles of white wine are mentioned in one box. The wedding day is generally even; The ceremonies of the emperor and the state are used in pairs. And three is singular, which is not suitable for some occasions.

In traditional etiquette, "three sacrifices" (usually chicken, fish and meat) are used for sacrifice, and three glasses of wine are served to the deceased at the funeral; After autumn, the last meal of the prisoner who asked for beheading before execution was also a three-course meal, which was called "beheading dish" by the people. Therefore, it is emphasized that "there are no three dishes on the table", but no specific three dishes are specified.

At a banquet in Shandong, if the waiter is too busy and only three dishes are served on the table, no matter who has something important, the host will not open the table. He must wait until the fourth course before starting to drink, so as to win prizes in the four seasons, harvest crops, and prosper six animals.