The Tai language of Qui Châu (northern Nghê An, Vietnam) was until recently one of the least known languages of the Daic phylum. It has an alphabetic orthography which is written vertically in columns from right to left, like traditional Chinese books. The present article, based on materials left by Henri Maspero, gives an overwiew of this writting system, which is ultimately derived from systems used in Siam in the XIVth Century. The Tai language of Qui Châu has a stratum of vocabulary closely related to the Yay branch of southern China. Our hypothesis is that these Tai people represent a Yay group who were forced out of southern China, settled in the Qui Châu area, and were later submerged by the migration of other Tai groups from northern Laos.