Nghệ An Province in the context of Houaphan province

Tonkin, also spelled Tongkin, Tonquin or Tongking, is an exonym referring to the area surrounding the Red River Delta of Northern Vietnam. During the 17th and 18th centuries, this term referred to the domain of Đàng Ngoài, under the control of the Trịnh lords, encompassing both the Northern and Thanh–Nghệ regions, north of the Gianh River at the 17.7th parallel. From 1884 to early 1945, the term referred to the French protectorate of Tonkin, which comprised only the Northern region, approximately north of the 20th parallel.

Bắc Trung Bộ (literally North Central Region, and often translated as North Central Coast) is one of the geographic regions of Vietnam. It consists of six provinces in northern part of Central Vietnam: Thanh Hóa, Nghệ An, Hà Tĩnh, Quảng Bình, Quảng Trị, Huế. The last two provinces were the northernmost provinces of State of Vietnam and South Vietnam until Reunification of Vietnam in 1976. In the Nguyễn dynasty, this area (except Thừa Thiên) was known as Hữu Trực Kỳ (the area located in the left of Thừa Thiên).

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 The name Thổ, which means "autochthonous" was originally applied to the Tày ethnic group, however this usage is obsolete.