Caolan is spoken by the Cao Lan-Sán Chay people of Tuyên Quang Province. According to the people, the Cao Lan and Sán Chay peoples had arrived from southern China 400 years ago together as one group, even though they spoke two different languages. Notably, they both use Chinese characters to record their languages. Smaller numbers are also found in the following provinces (Gregerson & Edmondson 1998).
The Sán Chay speak a form of Han Chinese, while the Cao Lan speak a Tai language. Gregerson & Edmondson (1998) considers Caolan to have a combination of both Northern Tai and Central Tai features. Like the E language of northern Guangxi, Cao Lan also displays influences form Pinghua, a Chinese vernacular spoken in Guangxi, China. Haudricourt (1973) believes that the Cao Lan may have adopted a form of Tai when they had stopped in Guangxi during their historical southward migration. The Chinese-speaking Sán Chay, on the other hand, might have been a Yao (or Mienic-speaking) group, as the Yao of Fangcheng, Guangxi call themselves san˧ tɕai˧. Today, the Chinese-speaking Sán Chay live mostly in Quảng Ninh, whereas the Cao Lan are mostly concentrated in Tuyên Quang, Thái Nguyên, and Bắc Giang.
Nguyễn Nam Tiến (1975). "Lại bàn về mối quan hệ giữa hai nhóm Cao Lan - Sán Chỉ". In, Ủy ban khoa học xã hội Việt Nam: Viện dân tộc học. Về vấn đề xác định thánh phần các dân tộc thiểu số ở miền bắc Việt Nam, 274-286. Hà Nội: Nhà xuất bản khoa học xã hội.