The term Thraco-Illyrian refers to a hypothesis according to which the Daco-Thracian and Illyrian languages comprise a distinct branch of Indo-European. Thraco-Illyrian is also used as a term merely implying a Thracian-Illyrian interference, mixture or sprachbund, or as a shorthand way of saying that it is not determined whether a subject is to be considered as pertaining to Thracian or Illyrian. Downgraded to a geo-linguistic concept, these languages are referred to as Paleo-Balkan.

The linguistical hypothesis was especially current in the early 20th century, but after the 1960s it was seriously called into question. New publications argued that no strong evidence for Thraco-Illyrian exists, and that the two language-areas show more differences than correspondences (Vladimir Georgiev, Ivan Duridanov, Eric Hamp, et al.). It has also been pointed out that the onomastic studies carried out in the 20th century were conducted through pan-Thracian and pan-Illyrian theoretical approaches, which are considerd outdated in current historical linguistics.^[1]

Linguistic contact and similarity

The Vardar, South Morava and Great Morava rivers are generally considered to approximate the border between the Illyrian and Thracian spheres, in the west and east respectively.^[2] However, Thracian and Illyrian did not have a clear-cut frontier.^[3] There was also, clearly, significant interaction between the Illyrian and Thracian spheres, with some Thracian groups occupying the Illyrian sphere and vice versa; the identity of some groups as Illyrian or Thracian has also remained unclear,^[4] or, in some instances, a Thraco-Illyrian mix.^[5] Such factors reinforce the impression that many similarities between the Illyrian and Thracian lexes resulted from language contact.^[6]

Other scholars, such as Romanian linguist and historian Ion Russu, argue that there were major similarities between Illyrian and Thracian and so a shared, ancestral linguistic branch is probable, rather than them forming a sprachbund.^[3] Among the Thraco-Illyrian correspondences noted by I. I. Russu are the following:

Not many Thraco-Illyrian correspondences are definite, and a number may be incorrect, even from the list above. Sorin Paliga (2002) however states: "According to the available data, we may surmise that Thracian and Illyrian were mutually understandable, e.g. like Czech and Slovak, in one extreme, or like Spanish and Portuguese, at the other."

Other linguists however argue that Illyrian and Thracian were different Indo-European branches which later converged through contact. It is also of significance that Illyrian languages still have not been classified whether they were centum or satem language, while it is undisputed that Thracian was a satem language by the Classical Period (the satem nature of proto-Thracian is disputed, Olteanu 2002).

Due to the fragmentary attestation of both Illyrian and Thraco-Dacian, the existence of a Thraco-Illyrian branch remains controversial. Evidence of a Thraco-Illyrian branch has also been sought in the Albanian language, which has been claimed to have developed from either an Illyrian language with Thraco-Dacian influences or a Thraco-Dacian language with Illyrian influences.^[7][8] Due to the paucity of written evidence, what can be said with certainty in current research is that on the one hand a significant group of shared Indo-European non-Romance cognates between Albanian and Romanian indicates at least contact with the 'Daco-Thraco-Moesian complex', and that on the other hand there is some evidence to argue that Albanian is descended from the 'Illyrian complex'.^[9] On the basis of shared features and innovations, Albanian is grouped together with Messapic in the same branch in the current phylogenetic classification of the Indo-European language family, called 'Albanoid' or 'Illyric'.^{[10][11][12][13][14]}