Xishuangbanna situates in the district where the ancient Chinese called "baiyue (100 minority tribes)", the minorities there, include the Dai minority, all have no characters before the 13th and the 14th century. Li Jing of Yuan Dynasty recorded in his book "Records of Yunnan": "the tribes in Xishuangbanna have no characters, and they keep records by carving on wood". This situation continued to the modern time. Each family of the Jinuo minority has many small bamboo chips. If they carve a small mark on it, it means "1"; when the marks total to "10", they would carve a big mark. These marks represents the amount of money, feet and kilograms etc; if they want to record the weight of objects, they would tie an object to the mark, such as cotton, corn and peanuts etc. to indicate what they mean and what's the weight, then, they would cut the chip into two, and it becomes a contract like our invoices; which could not be counterfeited.
Since they have no characters, it is very difficult for them to pass all messages; therefore, some minorities there created the "leave letters", in this way, they could convey more simple messages. The Kemu minority is an ethic group lives in the rain forests in the Sino-Laos borders, they would send "leave letters" to their relatives before important occasions. After the relatives or friends are in receipt of the letters, no matter what they are doing and how far away they are, they must put things at hand aside and to attend the occasion. If a banana leave wraps a bark, tobacco and hen feathers, it means that the man has died away, and the relatives are requested to show sympathy; if a banana leave wraps the bark, tobacco, hen feather, pepper and charcoal, it indicates an alarm of war. Besides, inside the letter or outside the letter, there is also carved bamboo chips, i.e. if a man places the leave letter in the appointed place, his love, based on the type and number of the leaves, would know where and when they would meet. If one of the Dai youths changed his (her) mind after falling into love, the other youth would use a banana leave to wrap a piece of black thread or charcoal with pepper and would ask someone else to sent to the leave letter to the other party. This means that he would make a thorough break. The black thread or charcoal means the heart of the other party has turned black, and the pepper means the other party is murderous. The flowers, fruits and roots of all plants in the "leave letters" all have certain indications; the people inside the minority or certain groups of people are fully aware their meaning. They are just like the codes of telegraphs used by the present people, and are very difficult for outsiders to read them.