This text examines the refl ection of the vowel yat in Serbian charters and letters from the noble chancelleries of the Sankovićs, Pavlovićs, and Sandalj Hranić Kosača, aiming to establish the relationship of these texts with the autochthonous vernacular. The research shows that the substitution of yat in the analyzed documents is systematic (ijekavian, ikavian, and ijekavian-ikavian), and that most of the scribes involved are indigenous. The infl uence of any potential pan-state literary koine is not registered – fewer than a third of the scribes are ikavian newcomers from the areas west of the Neretva and Bosnia rivers.