Guillaume Jacques is a French linguist who specializes in the study of Sino-Tibetan languages: Old Chinese, Tangut, Tibetan, Gyalrongic and Kiranti languages. He also performs research on the Algonquian and Siouan language families and publishes about languages of other families such as Breton. His case studies in historical phonology are set in the framework of panchronic phonology, aiming to formulate generalizations about sound change that are independent of any particular language or language group.

Sino-Tibetan, also cited as Trans-Himalayan in a few sources, is a family of more than 400 languages, second only to Indo-European in number of native speakers. Around 1.4 billion people speak a Sino-Tibetan language. The vast majority of these are the 1.3 billion native speakers of Sinitic languages. Other Sino-Tibetan languages with large numbers of speakers include Burmese and the Tibetic languages. Other languages of the family are spoken in the Himalayas, the Southeast Asian Massif, and the eastern edge of the Tibetan Plateau. Most of these have small speech communities in remote mountain areas, and as such are poorly documented.