The Red River, also known as the Hong River and Sông Cái in Vietnamese, and the Yuan River in Chinese, is a 1,149-kilometer (714 mi)-long river that flows from Yunnan in Southwest China through northern Vietnam to the Gulf of Tonkin. According to C. Michael Hogan, the associated Red River Fault was instrumental in forming the entire South China Sea at least as early as 37 million years before present. The name red and southern position in China are associated in traditional cardinal directions. The river is relatively shallow, and carries a lot of reddish silt along its way, appearing red brown in colour.

Vietnamese is an Austroasiatic language spoken primarily in Vietnam where it is the national and official language. Vietnamese is spoken natively by around 85 million people, several times as many as the rest of the Austroasiatic family combined. It is the native language of the Vietnamese (Kinh) people, as well as a second or first language for other ethnic groups in Vietnam.