The Thổ ethnic group (also Keo, Mon, Cuoi, Ho, Tay Poong) inhabits the mountainous regions of northern Vietnam, mainly Nghệ An Province southwest of Hanoi. Many Thổ speak the Tho language, which is closely related to Vietnamese.
The Thổ population numbered 91,430 in 2019.
The Thổ are one of the 4 main groups of Vietic speakers in Vietnam, the others being the Việt, Mường, and Chứt. The name Thổ, which means "autochthonous" was originally applied to the Tày ethnic group, however this usage is obsolete.
The Thổ people are a heterogeneous mix of different Vietic peoples. Around the end of the 17th century, Vietnam experienced multiple social upheavals that caused multiple migrations of Viet and Muong peoples into territory of other Vietic speaking ethnic minorities such as the Cuối and intermixed with the local populations. After a period of evolution, were given the name Thổ. Their culture combines elements of Viet and Muong culture, with elements of Thái culture. Their clothing is a mix of Vietnamese, Muong, and Tai traditions from the early modern period. Some women wear clothing similar to the Áo bà ba or the Áo tứ thân, but with a Sarong and a Kerchief. Men wear simple tunics and pants. Just like with the Muong, many clothing items are purchased from the Thái. They mainly grow rice and ramie, and use hemp to make bags, hammocks and nets. In the northern communes of Nghệ An, the stilt houses are identical to that of the Muong. In the South, however, they are in the Thái style.
Thổ consists of various different ethnolinguistic groups. Quán Vi Miên (2013:12) lists the following branches.
Họ (Lao/Thai exonym for the Han Chinese)
Kẹo (Keo) (Lao/Thai exonym for ethnic Kinh or Vietnamese)
Mọn (Mol) (Mường autonym)
Cuối
Đan Lai
Ly Hà
Tày Poọng (Phoọng, Poọng, Phống) (Lao exonym for the "Khạ", or Mon-Khmer hill tribes)
Thổ is a heterogeneous group; thus, local groups have distinct languages. However, all of Thổ languages belong to Vietic branch . Nguyễn Hữu Hoành (2009) classify Thổ languages into 5 groups base on their position in Vietic branch.