Sociolinguistic study of all Greek private letters on papyri and ostraca (approx 4,000-5,000), with focus on (1) the letter formulas and (2) their language. Both topics have a distinctive value in letters: (1) Formulas make up a substantial part of letters; the project consists of a lexical and syntactic investigation of these formulas, and a diachronic examination of the epistolary usage reflected in the standard formulas. (2) Private letters are the best source of information for the so-called everyday koinè (literally, the “common” form of the Greek language from the Hellenistic period onwards); hence, the standard formulas are the starting point for an investigation of the lexical and syntactic variation in everyday koinè; more specifically, sociolects, registers and bilingualism will be discerned on the basis of letters preserved in archives. The methodological framework combines corpus linguistics and sociolinguistics/pragmatics. The scientific contribution is threefold: (1) compiling a corpus of private letters, highly useful for future research. This could include a refinement of the labels currently classifying letters; (2) describing and explaining the diachronic evolution in letter formulas and the epistolary usus; (3) analysing sociolects, registers, and bilingualism in the koinè on the basis of the epistolary formulas of archival letters.