The article treats the problem of the language of the Slavic population between the Danube and the Adriatic Sea at the turn of the first millennium AD and its differentiation from the language of other groups of Slavs. The first part focuses on theoreti- cal foundations for linguistics in conjunction with other disciplines to establish a reasonably reliable picture of the language spoken by these populations and how to refer to this language. The second part builds on this foundation and illuminates the linguistic evidence for the relevant space and time. The interpretation of the material demonstrates that by the time of the turn of the millennium there had already been irreversible, exclusive linguistic innovations on the basis of which it is necessary to view the population at the relevant place and time as a community whose language had already begun to differentiate from other Slavic communities. The evidence for this is based on a small bundle of isoglosses that in the following centuries increased, so that by the time of the formation of standard languages this area could already be conceived of as the linguistic border of the Slovene language.Članek obravnava vprašanje jezika slovanskih prebivalcev ozemlja med Donavo in Jadranom na prelomu prvega tisočletja in njegove razmejitve do jezika preostalih Slovanov. Prvi del se osredinja na teoretične podlage, ki jezikoslovnim in interdisciplinarni prizadevanjem omogočajo relativno zanesljiva spoznanja o tem, kakšen jezik so govorili ti prebivalci in kako ga imenovati, drugi pa s temi izhodišči osvetljuje jezikovno gradivo obravnavanega prostora in časa. Interpretacija gradiva pokaže, da so se do preloma prvega tisočletja tu že izvršile ireverzibilne ekskluzivne jezikovne inovacije, zaradi katerih je treba prebivalce obravnavanega prostora in časa pojmovati kot identitetno skupnost, katere jezik se je od jezika sosednjih slovanskih identitetnih skupnosti že razlikoval po tankem snopu izoglos, ki se je v naslednjih stoletjih krepil, tako da je bil v času oblikovanja knjižnih standardov upravičeno razumljen kot jezikovna meja slovenskega jezika
