A written language developed on Belorussian territory at an early stage. In the twelfth and thirteenth centuries the tradition of copying manuscripts was carried out in such centres as Polack (Polotsk) and Turaü (Turov), but the language of these was Church Slavonic. It was only from the fourteenth century that vernacular elements began to appear in texts of Belorussian provenance, while the establishment of Belorussian as a literary language belongs to the fifteenth and sixteenth centuries, when its status was greatly enhanced by its adoption as the official language of the Grand Duchy. During this period the orthographical and grammatical norms of Old Belorussian were established, despite a tendency to preserve tradition al Church Slavonic-influenced forms, both in spelling and morphology. Thus already in the orthography of fourteenth-century documents we can discern such characteristic features of Belorussian pronunciation as the change of initial pre-consonantal [v I to [u I; the use of fricative [y I (plosive [gI was represented hy the digraph rK); the depalatalization of [z'), W], [5'], [c'] and [r']; and the clusters tri], [li] in place ofProto-Slavonic liquid + ·b. However, the most salient feature of Belorussian vowel phonology, akanne (the pronunciation of unstressed [0) as [a]), was reflected in the

orthography only sporadica11y at this time and even two centuries later its reflection remained inconsistent. The fifteenth and sixteenth centuries also saw much innovation in Belorussian lexis. The principal source of loanwords was Polish, which, since it also served as the medium for the introduction into Belorussian of loan-words from Latin and the western European languages, played an impartant role in the expansion of the vocabulary of Belorussian at this period in its history.