In the 17th and 18th centuries, Vietnam was ruled by a figurehead emperor of the Lê Dynasty.
Actual power rested in the Trịnh lords in the North,
called Đàng Ngoài (Outer Expanse) and Nguyễn lords in the South,
called "Đàng Trong" (Inner Expanse).
The two sides ruled their own domain independent of the other, and frequently fought each other.
The imposed separation encouraged the two regions to develop their own cultures.