Gianh River in the context of "Cochinchina"

Đàng Trong (chữ Nôm: 唐冲, lit. "Inner Circuit"), also known as Nam Hà (chữ Hán: 南河, "South of the River"), was the region of Vietnam south of the Gianh River, under the lordship of the Nguyễn clan, and later expanded through Vietnamese southward expansion. It was bordered to the north by Đàng Ngoài, ruled by the Lê–Trịnh.

Throughout the 17th century and most of the 18th century, the Nguyễn lords, though claiming loyalty to the Lê emperors in Thăng Long (Hanoi), ruled Đàng Trong as a de facto independent kingdom. Nguyễn rulers titled themselves as Chúa (chữ Nôm: 主, lit. "Lord") instead of Vua (chữ Nôm: 𤤰, lit. "King") until Lord Nguyễn Phúc Khoát officially claimed the title Vũ Vương (chữ Nôm: 武王, lit. "Martial King") in 1744.

Đàng Ngoài (chữ Hán: 唐外, lit. "Outer Land"), also known as Bắc Hà (北河, "North of the River") or Kingdom of Annam (安南國) by foreigners, was an area in northern Đại Việt (now Vietnam) during the 17th and 18th centuries as the result of Trịnh–Nguyễn War. The name Đàng Ngoài was first recorded in the Dictionarium Annamiticum Lusitanum et Latinum by Alexandre de Rhodes.

Đàng Ngoài was de facto ruled by the Trịnh lords with the Lê emperors acting as titular rulers. The capital was Thăng Long (now Hanoi). Thăng Long was also known as Đông Kinh 東京, meaning "Eastern Capital", from which the common European name for Đàng Ngoài "Tonkin" originated. It was bordered by Đàng Trong (under the Nguyễn lords) along the Gianh River in Quảng Bình province. The names gradually fell into disuse after Nguyễn Ánh reunified Đại Việt.

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.