The linguistic classification of the ancient Thracian language has long been a matter of contention and uncertainty, and there are widely varying hypotheses regarding its position among other Paleo-Balkan languages. It is not contested, however, that the Thracian languages were Indo-European languages which had acquired satem characteristics by the time they are attested.
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The longer Thracian inscriptions that are known (if they are indeed examples of Thracian sentences and phrases, which has not been determined) are not apparently close to Baltic, Slavic, Albanian, or any other known language, and they have not been satisfactorily deciphered aside from perhaps a few words.
Daco-Thracian
A Daco-Thracian (or Thraco-Dacian) grouping with Dacian was widely held until the 1950s, but is untenable (according to J. P. Mallory) in light of toponymic evidence: only a percent of place names north of the Danube betray "pan-Thracian" roots.
In the 1950s, the Bulgarian linguist Vladimir I. Georgiev published his work which argued that Dacian and Albanian should be assigned to a language branch termed Daco-Mysian, Mysian (the term Mysian derives from the Daco-Thracian tribe known as the Moesi) being thought of as a transitional language between Dacian and Thracian. Georgiev argued that Dacian and Thracian are different languages, with different phonetic systems, his idea being supported by the placenames, which end in -dava in Dacian and Mysian, as opposed to -para, in Thracian placenames. A series of authors favors Georgiev's view. Nevertheless, Polome hesitates to accept it. Crossland considers this seems to be a divergence of a Thraco-Dacian language into northern and southern groups of dialects, not as different as to rank as separate languages .
Thraco-Illyrian
Thraco-Illyrian is a hypothesis that the Thraco-Dacian 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 rivers Vardar and Morava are generally taken as the rough line of demarcation between the Illyrian sphere on the west and Thracian on the east. There is, however, much interference in the area between Illyrian and Thracian, with Thracian groups inhabiting Illyrian lands (the Thracian Bryges for example) and Illyrian groups overlapping into the Thracian zone (the Dardani seem to be a Thraco-Illyrian mix; Wilkes, 1992 et al.). It appears that Thracian and Illyrian do not have a clear-cut frontier. Similarities found between the Illyrian and Thracian lexis can thus be seen as merely linguistic interference.
Others such as I.I. Russu argue that there should have been major similarities between Illyrian and Thracian, and a common linguistic branch (not merely a Sprachbund) is probable. Among the Thraco-Illyrian correspondences Russu considers are the following:
Not many Thraco-Illyrian correspondences are definite, and a number may be incorrect, even from the list above. However, Sorin Paliga 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 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.
Due to the fragmentary attestation of both Illyrian and Thraco-Dacian, the existence of a Thraco-Illyrian branch remains controversial. In fact, this linguistic hypothesis was seriously called into question in the 1960s. New publications argued that no strong evidence for Thraco-Illyrian exists, and that the two language-areas show more differences than correspondences. The place of Paeonian language remains unclear. Modern linguists are uncertain on the classification of Paeonian, due to the extreme scarcity of materials we have on this language. On one side are Wilhelm Tomaschek and Paul Kretschmer, who claim it belonged to the Illyrian family, and on the other side is Dimiter Dechev, who claims affinities with Thracian.
Albanian
A hypothesis that the Thracian and the Albanian language together form a branch of the Indo-European language remains one of the major current theories. There are a number of close cognates between Thracian and Albanian, but this may indicate only that Thracian and Albanian are related but not very closely related satem IE languages on their own branches of Indo-European, analogous to the situation between Albanian and the Baltic languages: Albanian and Baltic share many close cognates, but the languages themselves are on different Indo-European satem language branches. There have been great changes in the Albanian language since Thracian times, and it remains difficult to demonstrate that Albanian and Thracian were any closer than Albanian and for example, Baltic. Still, the hypothesis that Thracian and Albanian form a distinct branch (often in these scenarios, along with Dacian) of Indo-European is given much consideration even today. A few of the cognates between Thracian and Albanian may actually represent borrowings from one language to another; in most cases this is ruled out because a word or lexical item follows the sound-changes expected in the language from its PIE sound-changes.
Among the cognates between Thracian and Albanian: the Thracian inscription mezenai on the Duvanli gold ring has been unanimously linked to Messapian menzana (=horse deity) to Albanian mëz (=pony), as well as to Romanian mânz (=colt), and it is agreed that Thracian mezenai meant 'horseman'; Thracian manteia is supposed to be cognate to Albanian mand (=mulberry).
Balto-Slavic
A hypothesis that Thracian (or in other scenarios, Daco-Thracian) and the Baltic languages or the Balto-Slavic languages form one branch of Indo-European has also been proposed. Here again due to the scanty evidence, though many close cognates exist between Balto-Slavic and Thracian, there is not enough evidence to demonstrate that Thracian and Balto-Slavic or Thracian and Baltic (excluding Slavic in some scenarios) form one branch of Indo-European.
Ancient Greek
Sorin Mihai Olteanu, a Romanian linguist and Thracologist, proposed that the Thracian (as well as the Dacian) language was a centum language in its earlier period, and developed satem features over time. One of the arguments for this idea is that there are many close cognates between Thracian and Ancient Greek. There are also substratum words in the Romanian language that are cited as evidence of the genetic relationship of the Thracian language to ancient Greek and the Ancient Macedonian language (the extinct language or Greek dialect of ancient Macedon). The Greek language itself may be grouped with the Phrygian language and Armenian language, both of which have been grouped with Thracian in the past.
As in the case with Albanian and Balto-Slavic, there is no compelling evidence that Thracian and Greek (or Daco-Thracian and Greco-Macedonian) share a close common ancestor.