This project aims to explore the history of the languages of the Basque Country (French, Castilian, Basque) through the study of ego-documents. It has two main objectives: (i) to establish a large corpus of ego-documents from the Basque linguistic area from Bilbao to Bayonne and covering the period 1680-1780; (ii) to provide an analysis of the ego-documents (French, Castilian, Basque) through the prism of the history of writing and historical linguistics and sociolinguistics. The research axes structuring the project are as follows: (i) the distribution of communicative and writing functions between French and Basque, and Castilian and Basque; (ii) the writing process in Basque, French and Castilian; (iii) the use and circulation of these ego-documents in their archival and institutional context as legal evidence, procedural documents or information tools; The working hypotheses are as follows: (i) the development of "an ordinary practice of writing" in eighteenth-century France (Roche 1985) also benefited the Basque language at a time when it was still spoken across different social classes. In the Spanish Basque Country, where the elite abandoned the use of Basque in the 18th century, ordinary Basque writing is less developed; (ii) the Basque language employed in these letters varies according to the profile of the writer, his geographical origin, his social background and his experience with Basque writing; (iii) the local French language spoken in Basque-speaking areas is clearly influenced by the Basque language, whatever the social background of the speakers; (iv) The Castilian used in these ego-documents has particular features attributable to its contact with Basque. This is most evident in the texts of less literate writers from lower ranks.