The project investigates the emergence and development of registers by examining the beginning of written transmission in Lithuanian and Latvian, two closely related languages in which register questions have not been investigated before. We focus on the two pre-eminent pioneers of the new literary languages, Johannes Bretke (1536–1602) for Old Lithuanian and Georg Mancelius (1593–1654) for Old Latvian, whose works attest a range of text types where intra-individual variation can be observed – postils (collections of pericopes and homilies) figuring prominently among these. Crucial is the interaction with (a) the written languages that provided translation sources and (b) the languages spoken in the multi-lingual environment. The following research questions arise: (i) How do registers emerge, and to what extent do individual authors create them? (ii) How can the influence of loan registers be evaluated? (iii) How can the influence of source languages within a multi-lingual setting be identified? (iv) How do established registers change over time?