The Phunoy are a Tibeto-Burmese population group in Phongsaly Province that has long been considered acculturated because of its adoption of various features of neighboring Tai societies, particularly Buddhism. This pioneering ethnography examines the Phunoy’s supposed acculturation and independent identity, demonstrating how the Phunoy emerged as a group and constructed a “mirroring” relationship with the various Tai and Lao realms dominating the region. As guardians of the borders and allies of the colonial authorities who administered the province, they progressively formed a territory where they established themselves as indispensable intermediaries between state power and the other mountain ethnic groups. The integration of the Phunoy continues within Lao society today and is part of the history of the stabilization of the margins in northern mainland Southeast Asia.