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The Sán Dìu (also known as San Deo, Trai, Trai Dat and Man Quan Coc; Chinese: 山由族; pinyin: Shān yóu zú; Cantonese Yale: Sanyau Juk; Chữ nôm: 𠊛山由; Quốc Ngữ: Người Sán Dìu) is a Yao ethnic group in northern Vietnam. They are believed to have migrated from Guangdong around 1600.
The group's estimated population as of 2000 was 117,500; a 2009 estimate put the number at 146,821. They speak an archaic variant of Cantonese, and it is suggested that some still speak Iu Mien. The major religions are Mahayana Buddhism and Taoism, with elements of animism and ancestor-worship. About 400 are Roman Catholics; a few are evangelical Protestants. This ethnic group is mainly concentrated in Quảng Ninh Province.
In the Chinese script, Sán Dìu is written as "山瑶", meaning the Yao in the Mountains. The Yao are an ethnic group living in southern China.