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.
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.
A Daco-Thracian (or Thraco-Dacian) grouping with Dacian as either the same language or different from Thracian 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. The hypothesis of a Thraco-Dacian or Daco-Thracian branch of IE, indicating a close link between the Thracian and Dacian languages, has numerous adherents, including Russu 1967, Georg Solta 1980, Vraciu 1980, Crossland, Trask (2000), McHenry (1993), Mihailov (2008). Crossland (1982) considers that the divergence of a presumed original Thraco-Dacian language into northern and southern groups of dialects is not so significant as to rank them as separate languages. According to Georg Solta (1982), there is no significant difference between Dacian and Thracian. Rădulescu (1984) accepts that Daco-Moesian possesses a certain degree of dialectal individuality, but argues that there is no fundamental separation between Daco-Moesian and Thracian. 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.[dubious ] Polomé (1982) considers that the evidence presented by Georgiev and Duridanov, although substantial, is not sufficient to determine whether Daco-Moesian and Thracian were two dialects of the same language or two distinct languages.
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. Georgiev argues that the distance between Dacian and Thracian was approximately the same as that between the Armenian and Persian languages. The claim of Georgiev that Albanian is a direct recent descendant of Daco-Moesian, not only a part of the branch, is highly based on speculations as suffixes from Dacian toponyms as Dava, for example, are lacking in modern Albanian toponymy (with one exception).
The Baltic classification of Dacian and Thracian has been proposed by the Lithuanian polymath Jonas Basanavičius, referred to as "Patriarch of Lithuania", who insisted this is the most important work of his life and listed 600 identical words of Balts and Thracians and was the first to investigate similarities in vocal traditions between Lithuanians and Bulgarians. He also theoretically included Dacian and Phrygian in the related group, but a part of this inclusion was unsupported by other authors, such as the linguistic analysis of Ivan Duridanov, which found Phrygian completely lacking parallels in either Thracian or Baltic languages.
The Bulgarian linguist Ivan Duridanov, in his first publication claimed that Thracian and Dacian are genetically linked to the Baltic languages and in the next one he made the following classification: "The Thracian language formed a close group with the Baltic (resp. Balto-Slavic), the Dacian and the "Pelasgian" languages. More distant were its relations with the other Indo-European languages, and especially with Greek, the Italic and Celtic languages, which exhibit only isolated phonetic similarities with Thracian; the Tokharian and the Hittite were also distant." Of about 200 reconstructed Thracian words by Duridanov, most cognates (138) appear in the Baltic languages, mostly in Lithuanian, followed by Germanic (61), Indo-Aryan (41), Greek (36), Bulgarian (23), Latin (10) and Albanian (8). The use of toponyms is suggested to determine the extent of a culture's influence. Parallels have enabled linguists, using the techniques of comparative linguistics, to decipher the meanings of several Dacian and Thracian placenames with, they claim, a high degree of probability. Of 300 attested Thracian geographic names most parallels were found between Thracian and Baltic and geographic names in the study of Duridanov. According to him the most important impression make the geographic cognates of Baltic and Thracian "the similarity of these parallels stretching frequently on the main element and the suffix simultaneously, which makes a strong impression". He also reconstructed Dacian words and Dacian placenames and found parallels mostly in the Baltic languages, followed by Albanian.  Other Slavic authors noted that Dacian and Thracian have much in common with Baltic onomastics and explicitly not in any similar way with Slavic onomastics, including cognates and parallels of lexical isoglosses, which implies a recent common ancestor.
After creating a list of names of rivers and personal names with a high number of parallels, the Romanian linguist Mircea M. Radulescu classified the Daco-Moesian and Thracian as Baltic languages, result of Baltic expansion to the south and also proposed such classification for Illyrian.
The Venezuelan-Lithuanian historian Jurate Rosales classifies Dacian and Thracian as Baltic languages.
The American linguist Harvey Mayer refers to both Dacian and Thracian as Baltic languages and refers to them as Southern or Eastern Baltic. He claims to have sufficient evidence for classifying them as Baltoidic or at least "Baltic-like," if not exactly, Baltic dialects or languages and classifies Dacians and Thracians as "Balts by extension". Mayer claims that he extracted an unambiguous evidence for regarding Dacian and Thracian as more tied to Lithuanian than to Latvian.
Finally, I label Thracian and Dacian as East Baltic...The fitting of special Dacian and Thracian features (which I identified from Duridanov’s listings) into Baltic isogloss patterns so that I identified Dacian and Thracian as southeast Baltic. South Baltic because, like Old Prussian, they keep unchanged the diphthongs ei, ai, en, an (north Baltic Lithuanian and Latvian show varying percentages of ei, ai to ie, and en, an to ę, ą (to ē, ā) in Lithuanian, to ie, uo in Latvian). East Baltic because the Dacian word žuvete (now in Rumanian spelled juvete) has ž, not z as in west Baltic, and the Thracian word pušis (the Latin-Greek transcription shows pousis which, I believe, reflects -š-.) with zero grade puš- as in Lithuanian pušìs rather than with e-grade *peuš- as in Prussian peusē. Zero grade in this word is east Baltic, e-grade here is west Baltic, while the other word for “pine, evergreen”, preidē (Prussian and Dacian), priede (Latvian), is marginal in Lithuanian matched by no *peus- in Latvian.
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:
|Abroi||Abre-||Abre- is an element taken from certain Thracian anthroponyms|
|Aploi, Aplus, Apulia||Apuli, Appulus, Apulum|
|Dardi, Dardani||Dardanos, Darda-para|
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.
In 1977 Georgiev claimed that Daco-Mysian was closely related to the Thracian branch of Indo-European and that Illyrian was different from Thracian "as much as Iranian from Latin" for example.
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, while according to Mayer, Albanian is a descendant of Illyrian and escaped any heavy Baltic influence of Daco-Thracian. 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). This view has not gained wide acceptance among scholars and is rejected by most Albanian linguists, who both mainly consider that Albanian belongs to the Illyrian branch of IE. Polome accepts the claim that Albanian is descended from Illyrian, not Thracian, although he considers the evidence for this as inconclusive. A toponymic analysis of a Bulgarian linguist showed incosistency between toponymy of the Bessi and Albanian toponymy.
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.
Thraco-Phrygian or Thraco-Armenian hypothesis
For a long time a Thraco-Phrygian hypothesis grouping Thracian with the extinct Phrygian language was considered, largely based on Greek historians like Strabo. By extension of identifying Phrygians with Proto-Armenians, a Thraco-Phrygian branch of Indo-European was postulated with Thracian, Phrygian and Armenian and constituent languages. The evidence for this seems to have been mostly based on interpretations of history and identifying the eastern Mushki with Armenians and assuming they had branched off from western Mushki (whom has been conclusively identified as Phrygians). However, in 1988 Fredrik Kortlandt argued, on linguistic grounds such as a common treatment of Proto-Indo-European glottal stops, that Armenian descended from a Thracian dialect. Thus, forming a Thraco-Armenian branch of Indo-European. In 2016 Kortlandt extended his theories, postulating a link between Thraco-Armenian and the hypothetical Graeco-Phrygian language family, despite Thracian and Armenian being Satem languages and Greek and Phrygian Centum languages Kortlandt identifies sound correspondences and grammatical similarities postulating a relationship between his Thraco-Armenian family and the more established Graeco-Phrygian family. Graeco-Armenian is by itself a common hypothesized subgrouping of Indo-European languages. Kortlandt considers Albanian a descendent of Dacian which he regards as belonging to a separate language family as Thraco-Armenian.
Older textbooks grouped Phrygian and Armenian with Thracian, but the belief is no longer popular and is mostly discarded. Today, Phrygian is not widely seen as linked to Thracian. Georgiev claimed that Thracian is different from Phrygian "as much as Greek from Albanian", comparing 150 Phrygian inscriptions. Duridanov found in 1976 Phrygian completely lacking parallels in Thracian and concluded that the Thraco-Phrygian theory is debunked. Duridanov argued that the Thraco-Illyrian theory is a mistake of the past: "In the past it was regarded that Thracian together with the Phrygian and other vanished languages belonged to the Iranian branch of the Indo-European languages. This mistake was corrected in the 80’s of the last century, but the ambiguities still persisted: the Thracian was combined in one group with the Phrygian (P. Kretschmer), and later – with the Illyrian (the language, spoken in the modern Dalmatia and Albania)."
- Balkan sprachbund
- Romanian words of possible Dacian origin (and comparison with Albanian words)
- Venetic language
- This is confirmed among others by Benjamin W. Fortson in his Indo-European Language and Culture, when he states that "all attempts to relate Thracian to Phrygian, Illyrian, or Dacian...are...purely speculative." (p. 90).
- Ilija Casule even links Thracian and Phrygian with the Burushaski language, a language isolate spoken in northern Pakistan.
- Duridanov, Ivan. The Language of the Thracians.
- Mallory, J. P. (1997). "Thracian language". In Mallory, J. P.; Adams, Douglas Q. Encyclopedia of Indo-European Culture. Taylor & Francis. p. 576.
- Crossland & 1982 838.
- The Moesi of Moesia are not to be confused with the Mysoi (Mysians) of Mysia in ancient Anatolia, though some[who?] hypothesize that the Mysians are directly descended from the Balkan Moesi. Georgiev claimed that Thracian is related to Daco-Moesian but distinct from Illyrian. This is hypothesized mostly on the basis of Strabo's claim that some Moesians had migrated to Mysia, becoming the Mysians of Anatolia. Also in some classical sources the Moesi of Moesia are called Μυσοί; Thracologists often see this as a corruption. Thracologists have noted a Thracian element in Mysia, but the Mysians are more often viewed as a non-Thraco-Dacic people akin to the Phrygians, not the Thracians.
- Vladimir Georgiev (Gheorghiev), Raporturile dintre limbile dacă, tracă şi frigiană, "Studii Clasice" Journal, II, 1960, 39-58.
- Dras. J. Basanavičius. Apie trakų prygų tautystę ir jų atsikėlimą Lietuvon
- Balts and Goths: the missing link in European history. Vydūnas Youth Fund.
- Daskalov, Roumen; Vezenkov, Alexander. Entangled Histories of the Balkans - Volume Three: Shared Pasts, Disputed Legacies. BRILL. ISBN 9789004290365.
- Vyčinienė, Daiva. RELATIONSHIPS BETWEEN LITHUANIAN AND BALKAN SCHWEBUNGS-DIAPHONIE: INTERDISCIPLINARY SEARCH KEY.
- Duridanov 1976.
- "Dėl žynio Žalmokšio vardo kilmės | Vydos Vartai Yogi.lt". www.yogi.lt (in Lithuanian).
- reconstructed Thracian words
- Thracian geographic names
- Duridanov 1985.
- ON. Trubachev, ‘Linguistics and ethnogenesis of the Slavs: the ancient Slavs as evi- denced by etymology and onomastics,’ journal oj'Inaeruropean Studies 13 (1985), pp. 203—256, here p. 215. On the other hand, certain isoglosses, particularly lexical ones, in Balkan Slavic languages have cognates in Baltic, but not in East Slavic languages. See D. Brozovic, ‘Doseljenje slavena i njihovi dodiri sa starosjediocima u svjetlu lingvistickih istraiivanja’ [The settlement of the Slavs and their contacts with the native population in the light of linguistic studies], in Simpozijum ‘Treaslavenski etnit‘lei elemenii na Balkanu u etnogenezi juinih Slovena”, odrian 24—26. oktobra 1968 u Mostaru [Symposium on Pre-Slavic ethnic elements on the Balkans and the ethnognesis of Southern Slavs; Mostar, 24–26 October 1968], ed. A. Benac (Sarajevo: Akademija nauka i umjetnosti Bosne i Hercegovine, 1969), 1313- 129—140, here pp. 151-152
- Radulescu M., The Indo-European position of lllirian, Daco-Mysian and Thracian: a historic Methodological Approach, 1987
- JŪRATĖ STATKUTĖ DE ROSALES EUROPOS ŠAKNYS IR MES, LIETUVIAI, pp. 43-70 (PDF).
- Mayer, H.E. (1992). "Dacian and Thracian as southern Baltoidic". Lituanus. Defense Language Institute, United States Department of Defense. 38 (2). ISSN 0024-5089.
- Mayer, H.E. (1996). "SOUTH BALTIC". Lituanus. 42 (2).
- Mayer, H.E. (1997). "BALTS AND CARPATHIANS". Lituanus. 43 (2).
- Mayer, H.E. (1999). "Dr. Harvey E. Mayer, February 1999".
- Encyclopædia Britannica - Balkans.
- Wilkes, J.J. The Illyrians. 1992, ISBN 0-631-19807-5, p. 85. "Whether the Dardanians were an Illyrian or a Thracian people has been much debated..."
- Russu (1969).
- Hemp, Georgiev, et al.
- Paliga, S. (2001–2002). "Pre-Slavic and Pre-Romance Place-Names in Southeast Europe". Orpheus (Sofia) 11–12: 85–132.
- The satem nature of proto-Thracian is disputed (Olteanu 2002).
- See works by Vladimir Georgiev, Ivan Duridanov, and Eric Hamp.
- Траките и техният език (1977 В Георгиев), p. 132, 183, 192, 204
- Vladimir Orel, A Concise Historical Grammar of the Albanian language; et al.
- Lloshi, 1999, p283
- Sorin Mihai Olteanu - The Thracian Palatal (Accessed: February 26, 2009).
- I. M. Diakonoff ”The Pre-History of the Armenian People” Erevan, 1968, English Translation by Lori Jennings (Delmar, New York, 1984) http://www.attalus.org/armenian/diakph10.htm
- Frederik Kortlandt ”Phrygian Between Greek and Armenian” Academie Bulgare des Sciences Linguistique Balkanique LV (2016), 2-3
- See C. Brixhe - Ancient languages of Asia Minor, Cambridge University Press, 2008 We will dismiss, at least temporarily, the idea of a Thraco-Phrygian unity. Thraco-Dacian (or Thracian and Daco-Mysian) seems to belong to the eastern (satem) group of Indo-European languages and its (their) phonetic system is far less conservative than that of Phrygian (see Brixhe and Panayotou 1994, §§3ff.
- Polomé 1982, pp. 887–888.
- Crossland, R.A.; Boardman, John (1982). "Linguistic problems of the Balkan area in the late prehistoric and early Classical period" in The Cambridge Ancient History Volume 3, Part 1. Cambridge University Press. ISBN 978-0-521-22496-3.
- Polomé, Edgar Charles (1982). "Balkan Languages (Illyrian, Thracian and Daco-Moesian)". Cambridge Ancient History. III.1. pp. 866–888.