Editing Indo-European languages (section)

== Classification ==
{{See also|List of Indo-European languages}}
The various subgroups of the Indo-European language family include ten major branches, listed below in alphabetical order:
* [[Albanian language|Albanian]], attested from the 13th century;<ref name="dictalit">{{cite encyclopedia |last=Elsie |first=Robert |author-link=Robert Elsie |title=Theodor of Shkodra (1210) and Other Early Texts |encyclopedia=Albanian Literature: A Short History |page=5 |publisher=[[I. B. Tauris]] |location=New York |date=2005}}</ref> [[Proto-Albanian]] evolved from an ancient [[Paleo-Balkan language]], traditionally thought to be [[Illyrian languages|Illyrian]], or otherwise a totally unattested Balkan [[Indo-European language]] that was closely related to Illyrian and [[Messapic]].<ref>In his latest book, [[Eric Hamp]] supports the thesis that the Illyrian language belongs to the Northwestern group, that the Albanian language is descended from Illyrian, and that Albanian is related to Messapic which is an earlier Illyrian dialect ({{harvnb|Hamp|2007}}).</ref><ref name="De Vaan">{{Cite book |last=De Vaan |first=Michiel |author-link=Michiel de Vaan |chapter=The phonology of Albanian |chapter-url=https://books.google.com/books?id=SuR8DwAAQBAJ&q=Ylli+Proto-Albanian&pg=PA1732 |title=Handbook of Comparative and Historical Indo-European Linguistics |editor-last=Klein |editor-first=Jared |editor-last2=Joseph |editor-first2=Brian |editor-last3=Fritz |editor-first3=Matthias |date=11 June 2018 |publisher=Walter de Gruyter |isbn=978-3-11-054243-1 |pages=1732–1749}}</ref><ref>{{cite book |last1=Curtis |first1=Matthew Cowan |title=Slavic–Albanian Language Contact, Convergence, and Coexistence |url=https://eric.ed.gov/?id=ED546136 |access-date=31 March 2017 |page=18 |quote=So while linguists may debate about the ties between Albanian and older languages of the Balkans, and while most Albanians may take the genealogical connection to Illyrian as incontrovertible, the fact remains that there is simply insufficient evidence to connect Illyrian, Thracian, or Dacian with any language, including Albanian |isbn=978-1-267-58033-7 |date=30 November 2011}}</ref>
* [[Anatolian languages|Anatolian]], extinct by [[Late Antiquity]], spoken in [[Anatolia]], attested in isolated terms in [[Luwian]]/[[Hittites|Hittite]] mentioned in Semitic [[Akkadian language|Old Assyrian]] texts from the 20th and 19th centuries&nbsp;BC, [[Hittite texts]] from about 1650&nbsp;BC.<ref>{{cite web |url=http://www.leidenuniv.nl/en/researcharchive/index.php3-c=178.htm |title=The peaks and troughs of Hittite |date=2 May 2006 |website=www.leidenuniv.nl |access-date=25 November 2013 |archive-date=3 February 2017 |archive-url=https://web.archive.org/web/20170203061604/http://www.leidenuniv.nl/en/researcharchive/index.php3-c=178.htm |url-status=dead}}</ref><ref>{{cite web |url=http://oi.uchicago.edu/pdf/ar/61-70/65-66/65-66_CHD.pdf |title=The Hittite Computer Analysis Project |first=Hans G. |last=Güterbock |access-date=25 November 2013 |archive-date=2 December 2013 |archive-url=https://web.archive.org/web/20131202224845/http://oi.uchicago.edu/pdf/ar/61-70/65-66/65-66_CHD.pdf |url-status=dead}}</ref>
* [[Armenian language|Armenian]], attested from the early 5th century&nbsp;AD. It evolved from the [[Proto-Armenian language]] which, according to the [[Armenian hypothesis]], developed ''in situ'' from the [[Proto-Indo-European language]] of the 3rd millennium&nbsp;BC.<ref>[[Tamaz V. Gamkrelidze|Gamkrelidze, Tamaz V.]]; [[Ivanov, Vyacheslav]] (1995). ''Indo-European and the Indo-Europeans: A Reconstruction and Historical Analysis of a Proto-Language and Proto-Culture. Part I: The Text. Part II: Bibliography, Indexes''. Walter de Gruyter. {{ISBN|978-3-11-081503-0}}.</ref><ref>Haber, Marc; Mezzavilla, Massimo; Xue, Yali; Comas, David; Gasparini, Paolo; Zalloua, Pierre; Tyler-Smith, Chris (2015). "Genetic evidence for an origin of the Armenians from Bronze Age mixing of multiple populations". ''European Journal of Human Genetics''. '''24''' (6): 931–936. [[bioRxiv]]&nbsp;10.1101/015396. [[Doi (identifier)|doi]]:10.1038/ejhg.2015.206. [[PMC (identifier)|PMC]]&nbsp;4820045. [[PMID]]&nbsp;26486470.</ref>
* [[Balto-Slavic]], believed by most Indo-Europeanists<ref>Such as {{harvnb|Schleicher|1874–1877|p=8}}, {{harvnb|Szemerényi|1957}}, {{harvnb|Collinge|1985}}, and {{harvnb|Beekes|1995|p=22}}.</ref> to form a phylogenetic unit, while a minority ascribes similarities to prolonged language-contact.
** [[Slavic languages|Slavic]] (from [[Proto-Slavic]]), attested from the 9th century&nbsp;AD ([[Pre-Christian Slavic writing|possibly earlier]]), earliest texts in [[Old Church Slavonic]]. Slavic languages include [[Bulgarian language|Bulgarian]], [[Russian language|Russian]], [[Polish language|Polish]], [[Czech language|Czech]], [[Slovak language|Slovak]], [[Silesian language|Silesian]], [[Kashubian language|Kashubian]], [[Macedonian language|Macedonian]], [[Serbo-Croatian]] ([[Bosnian language|Bosnian]], [[Croatian language|Croatian]], [[Montenegrin language|Montenegrin]], [[Serbian language|Serbian]]), [[Sorbian language|Sorbian]], [[Slovenian language|Slovenian]], [[Ukrainian language|Ukrainian]], [[Belarusian language|Belarusian]], and [[Rusyn language|Rusyn]].
** [[Baltic languages|Baltic]], attested from the 14th century; although attested relatively recently, they retain many archaic features attributed to [[Proto-Indo-European]] (PIE). Living examples are [[Lithuanian language|Lithuanian]] and [[Latvian language|Latvian]].
* [[Celtic languages|Celtic]] (from [[Proto-Celtic]]), attested since the 6th century&nbsp;BC; [[Lepontic]] inscriptions date as early as the 6th century&nbsp;BC; [[Celtiberian language|Celtiberian]] from the 2nd century&nbsp;BC; Primitive Irish [[Ogham inscription]]s from the 4th or 5th century&nbsp;AD, earliest inscriptions in [[Old Welsh]] from the 7th century&nbsp;AD. Modern Celtic languages include [[Welsh language|Welsh]], [[Cornish language|Cornish]], [[Breton language|Breton]], [[Scottish Gaelic]], [[Irish language|Irish]] and [[Manx language|Manx]].
* [[Germanic languages|Germanic]] (from [[Proto-Germanic]]), earliest attestations in [[runic]] inscriptions from around the 2nd century&nbsp;AD, earliest coherent texts in [[Gothic language|Gothic]], 4th century&nbsp;AD. [[Old English]] manuscript tradition from about the 8th century&nbsp;AD. Includes [[English language|English]], [[Frisian languages|Frisian]], [[German language|German]], [[Dutch language|Dutch]], [[Scots language|Scots]], [[Danish language|Danish]], [[Swedish language|Swedish]], [[Norwegian language|Norwegian]], [[Afrikaans]], [[Yiddish]], [[Low German]], [[Icelandic language|Icelandic]], [[Elfdalian]], and [[Faroese language|Faroese]].
* [[Hellenic languages|Hellenic]] (from [[Proto-Greek]], see also [[History of Greek]]); fragmentary records in [[Mycenaean language|Mycenaean]] Greek from between 1450 and 1350&nbsp;BC have been found.<ref>{{cite web |url=http://www.science20.com/news_articles/tablet_discovery_pushes_earliest_european_writing_back_150_years-77650 |title=Tablet Discovery Pushes Earliest European Writing Back 150 Years |website=Science 2.0 |date=30 March 2011}}</ref> [[Homer]]ic texts date to the 8th century&nbsp;BC.
* [[Indo-Iranian languages|Indo-Iranian]], attested {{Circa|1400&nbsp;BC|lk=no}}, descended from [[Proto-Indo-Iranian]] (dated to the late 3rd millennium&nbsp;BC).
** [[Indo-Aryan languages|Indo-Aryan]], attested from around 1400&nbsp;BC in [[Hittite language|Hittite]] texts from [[Anatolia]], showing [[Indo-Aryan superstrate in Mitanni|traces of Indo-Aryan]] words.<ref>{{cite book |title=Indian History |url=https://books.google.com/books?id=MazdaWXQFuQC&pg=SL1-PA114 |publisher=Allied Publishers |isbn=978-81-8424-568-4 |page=114 |date=1988}}</ref><ref>{{cite web |url=https://www.worldhistory.org/Mitanni/ |title=Mitanni |first=Joshua J. |last=Mark |date=28 April 2011 |website=[[World History Encyclopedia]]}}</ref> Epigraphically from the 3rd century&nbsp;BC in the form of [[Prakrit]] ([[Edicts of Ashoka]]). The [[Rigveda]] is assumed to preserve intact records [[Patha|via oral tradition]] dating from c.&nbsp;the mid-2nd millennium&nbsp;BC in the form of [[Vedic Sanskrit]]. Includes a wide range of modern languages from [[North India]], Eastern Pakistan and Bangladesh, including [[Hindustani language|Hindustani]] ([[Hindi]], [[Urdu]]), [[Bengali language|Bengali]], [[Odia language|Odia]], [[Assamese language|Assamese]], [[Punjabi language|Punjabi]], [[Kashmiri language|Kashmiri]], [[Gujarati language|Gujarati]], [[Marathi language|Marathi]], [[Sindhi language|Sindhi]] and [[Nepali language|Nepali]], as well as [[Sinhala language|Sinhala]] of [[Sri Lanka]] and [[Maldivian language|Dhivehi]] of the [[Maldives]] and [[Minicoy]].
** [[Iranian languages|Iranian]] or Iranic, attested from roughly 1000&nbsp;BC in the form of [[Avestan]]. Epigraphically from 520&nbsp;BC in the form of [[Old Persian]] ([[Behistun inscription]]). Includes [[Persian language|Persian]], [[Pashto]], [[Kurdish languages|Kurdish]], [[Balochi language|Balochi]], [[Luri language|Luri]], and [[Ossetian language|Ossetian]].
** [[Nuristani languages|Nuristani]], attested since the 20th century, are among the newest Indo-European languages to be studied. Includes [[Katë]], [[Wasi-wari|Prasun]], [[Askunu language|Ashkun]], [[Nuristani Kalasha]], [[Tregami]], and [[Zemiaki]].
* [[Italic languages|Italic]] (from [[Proto-Italic]]), attested from the 7th century&nbsp;BC. Includes the ancient [[Osco-Umbrian languages]], [[Faliscan language|Faliscan]], as well as [[Latin]] and its descendants, the [[Romance languages]], such as [[Italian language|Italian]] and [[French language|French]].
* [[Tocharian languages|Tocharian]], with proposed links to the [[Afanasevo culture]] of Southern Siberia.<ref>{{cite journal |first=David W. |last=Anthony |title=Two IE phylogenies, three PIE migrations, and four kinds of steppe pastoralism |journal=Journal of Language Relationship |volume=9 |date=2013 |pages=1–22 |doi=10.31826/jlr-2013-090105 |s2cid=132712913 |doi-access=free}}</ref> Extant in two dialects (Turfanian and Kuchean, or Tocharian A and B), attested during roughly the 6th–9th centuries&nbsp;AD. Marginalized by the Old Turkic [[Uyghur Khaganate]] and probably extinct by the 10th century.

In addition to the classical ten branches listed above, several extinct and little-known languages and language-groups have existed or are proposed to have existed:
* [[Ancient Belgian]]: hypothetical language associated with the proposed [[Nordwestblock]] cultural area. Speculated to be connected to Italic or Venetic, and to have certain phonological features in common with Lusitanian.<ref>F. Ribezzo, ''Revue Internationale d'Onomastique'', II, 1948 {{p.|43}} sq. et III 1949, {{p.|45}} sq., M.Almagro dans ''RSLig'', XVI, 1950, {{p.|42}} sq, P.Laviosa Zambotti, l.c.</ref><ref name="Bernard">{{cite book |last1=Bernard |first1=Sergent |title=Les Indo-Européens: Histoire, langues, mythes |date=1995 |publisher=Bibliothèques scientifiques Payot |location=Paris |pages=84–85 |language=fr}}</ref>
* [[Cimmerian language|Cimmerian]]: possibly Iranic, Thracian, or Celtic
* [[Dacian language|Dacian]]: possibly very close to Thracian
* [[Elymian language|Elymian]]: Poorly-attested language spoken by the [[Elymians]], one of the three indigenous (i.e. pre-Greek and pre-Punic) tribes of Sicily. Indo-European affiliation widely accepted, possibly related to Italic or Anatolian.<ref name="Olga">{{cite book |last1=Tribulato |first1=Olga |title=Language and Linguistic Contact in Ancient Sicily |date=December 2012 |publisher=[[Cambridge University Press]] |isbn=978-1-139-24893-8 |pages=95–114}}</ref><ref>{{cite book |last1=Price |first1=Glanville |title=Encyclopedia of the languages of Europe |date=April 2000 |publisher=[[John Wiley & Sons]] |isbn=0-631-22039-9 |page=136}}</ref>
* [[Illyrian languages|Illyrian]]: possibly related to Albanian, Messapian, or both
* [[Liburnian language|Liburnian]]: evidence too scant and uncertain to determine anything with certainty
* [[Ligurian language (ancient)|Ligurian]]: possibly close to or part of Celtic.<ref name=kruta1>{{cite book |last=Kruta |first=Venceslas |date=1991 |title=The Celts |publisher=Thames and Hudson |page=54}}</ref>
* [[Lusitanian language|Lusitanian]]: possibly related to (or part of) Celtic, Ligurian, or Italic
* [[Ancient Macedonian language|Ancient Macedonian]]: proposed relationship to Greek.
* [[Messapic]]: not conclusively deciphered, often considered to be related to Albanian as the available fragmentary linguistic evidence shows common characteristic innovations and a number of significant lexical correspondences between the two languages<ref>{{cite book |last=Trumper |first=John |chapter=Some Celto-Albanian isoglosses and their implications |editor1-last=Grimaldi |editor1-first=Mirko |editor2-last=Lai |editor2-first=Rosangela |editor3-last=Franco |editor3-first=Ludovico |editor4-last=Baldi |editor4-first=Benedetta |title=Structuring Variation in Romance Linguistics and Beyond: In Honour of Leonardo M. Savoia |year=2018 |publisher=John Benjamins Publishing Company |isbn=9789027263179 |url=https://books.google.com/books?id=kAR-DwAAQBAJ}} pp. 283–286.</ref><ref>{{cite book |last=Friedman |first=Victor A. |title=The Routledge Handbook of Language Contact |chapter=The Balkans |series=Routledge Handbooks in Linguistics |editor=[[Evangelia Adamou]], [[Yaron Matras]] |publisher=Routledge |year=2020 |isbn=978-1-351-10914-7 |pages=385–403 |url=https://books.google.com/books?id=x4rvDwAAQBAJ}} p. 388</ref><ref>{{cite journal |last=Friedman |first=Victor A. |title=The Balkan Languages and Balkan Linguistics |journal=Annual Review of Anthropology |volume=40 |year=2011 |pages=275–291 |doi=10.1146/annurev-anthro-081309-145932}}</ref>
* [[Paionian language|Paionian]]: extinct language once spoken north of Macedon
* [[Phrygian language|Phrygian]]: language of the ancient [[Phrygians]]. Very likely, but not certainly, a sister group to Hellenic.
* [[Sicel language|Sicel]]: an ancient language spoken by the Sicels (Greek Sikeloi, Latin Siculi), one of the three indigenous (i.e. pre-Greek and pre-Punic) tribes of Sicily. Proposed relationship to Latin or Proto-Illyrian (Pre-Indo-European) at an earlier stage.<ref>{{cite book |last=Fine |first=John |date=1985 |title=The ancient Greeks: a critical history |publisher=[[Harvard University Press]] |page=72 |isbn=978-0-674-03314-6 |quote=Most scholars now believe that the Sicans and Sicels, as well as the inhabitants of southern Italy, were basically of Illyrian stock superimposed on an aboriginal 'Mediterranean' population.}}</ref>
* [[Sorothaptic]]: proposed, pre-Celtic, Iberian language
* [[Thracian language|Thracian]]: possibly including Dacian
* [[Venetic]]: shares several similarities with Latin and the Italic languages, but also has some affinities with other IE languages, especially Germanic and Celtic.<ref>{{cite book |last1=Lejeune |first1=Michel |title=Manuel de la langue vénète |date=1974 |publisher=C. Winter |location=Heidelberg |page=341}}</ref><ref>{{cite book |last1=Pokorny |first1=Julius |title=Indogermanisches Etymologisches Wörterbuch |language=de |trans-title=Indogermanic Etymological Dictionary |date=1959 |location=Bern |publisher=Francke |pages=708–709, 882–884}}</ref>

[[File:Indo-European language tree (with major international languages highlighted).svg|thumb|upright=1.8|Indo-European family tree in order of first attestation]]
[[File:IndoEuropeanLanguageFamilyRelationsChart.jpg|thumb|upright=1.8|Indo-European language family tree based on "Ancestry-constrained phylogenetic analysis of Indo-European languages" by Chang et al.<ref name=chang />]]
Membership of languages in the Indo-European language family is determined by [[Genetic (linguistics)|genealogical]] relationships, meaning that all members are presumed descendants of a common ancestor, [[Proto-Indo-European]]. Membership in the various branches, groups, and subgroups of Indo-European is also genealogical, but here the defining factors are ''shared innovations'' among various languages, suggesting a common ancestor that split off from other Indo-European groups. For example, what makes the Germanic languages a branch of Indo-European is that much of their structure and phonology can be stated in rules that apply to all of them. Many of their common features are presumed innovations that took place in [[Proto-Germanic]], the source of all the Germanic languages.

In the 21st century, several attempts have been made to model the phylogeny of Indo-European languages using Bayesian methodologies similar to those applied to problems in biological phylogeny.<ref name=remco>{{cite journal |last1=Bouckaert |first1=Remco |last2=Lemey |first2=Philippe |date=24 August 2012 |title=Mapping the Origins and Expansion of the Indo-European Language Family |url= |journal=Science |volume=337 |issue=6097 |pages=957–960 |doi=10.1126/science.1219669 |pmid=22923579 |pmc=4112997 |bibcode=2012Sci...337..957B |hdl=11858/00-001M-0000-000F-EADF-A}}</ref><ref name=drinka>{{cite journal |last1=Drinka |first1=Bridget |author-link=Bridget Drinka |date=1 January 2013 |title=Phylogenetic and areal models of Indo-European relatedness: The role of contact in reconstruction |journal=Journal of Language Contact |volume=6 |issue=2 |pages=379–410 |doi=10.1163/19552629-00602009 |doi-access=free}}</ref><ref name=chang>{{cite journal |last1=Chang |first1=Will |last2=Chundra |first2=Cathcart |date=January 2015 |title=Ancestry-constrained phylogenetic analysis supports the Indo-European steppe hypothesis |url=https://www.linguisticsociety.org/sites/default/files/news/ChangEtAlPreprint.pdf |journal=[[Language (journal)|Language]] |volume=91 |issue=1 |pages=194–244 |doi=10.1353/lan.2015.0005 |s2cid=143978664 |access-date=30 September 2020}}</ref> Although there are differences in absolute timing between the various analyses, there is much commonality between them, including the result that the first known language groups to diverge were the Anatolian and Tocharian language families, in that order.

=== Tree versus wave model ===
{{See also|Language change}}
The "[[tree model]]" is considered an appropriate representation of the genealogical history of a language family if communities do not remain in contact after their languages have started to diverge. In this case, subgroups defined by shared innovations form a nested pattern. The tree model is not appropriate in cases where languages remain in contact as they diversify; in such cases subgroups may overlap, and the "[[wave model]]" is a more accurate representation.<ref>{{Citation |last=François |first=Alexandre |contribution=Trees, Waves and Linkages: Models of Language Diversification |editor1-last=Bowern |editor1-first=Claire |editor2-last=Evans |editor2-first=Bethwyn |title=The Routledge Handbook of Historical Linguistics |pages=161–189 |publisher=[[Routledge]] |place=London |year=2014 |isbn=978-0-415-52789-7 |contribution-url=http://alex.francois.free.fr/data/AlexFrancois_2014_HHL_Trees-waves-linkages_Diversification.pdf |ref=francois}}</ref> Most approaches to Indo-European subgrouping to date have assumed that the tree model is by-and-large valid for Indo-European;<ref>{{cite journal |title=From August Schleicher to Sergei Starostin: on the development of the tree-diagram models of the Indo-European languages |last=Blažek |first=Václav |journal=[[Journal of Indo-European Studies]] |year=2007 |volume=35 |issue=1–2 |pages=82–109}}</ref> however, there is also a long tradition of wave-model approaches.<ref>{{cite book |title=Les dialectes indo-européens |language=fr |trans-title=The Indo-European dialects |publisher=Honoré Champion |last=Meillet |first=Antoine |year=1908 |location=Paris}}</ref><ref>{{cite book |title=I dialetti indoeuropei |publisher=Paideia |last=Bonfante |first=Giuliano |year=1931 |location=Brescia}}</ref>{{sfn|Porzig|1954}}

In addition to genealogical changes, many of the early changes in Indo-European languages can be attributed to [[language contact]]. It has been asserted, for example, that many of the more striking features shared by Italic languages (Latin, Oscan, Umbrian, etc.) might well be [[areal features]]. More certainly, very similar-looking alterations in the systems of [[long vowel]]s in the West Germanic languages greatly postdate any possible notion of a [[proto-language]] innovation (and cannot readily be regarded as "areal", either, because English and continental West Germanic were not a linguistic area). In a similar vein, there are many similar innovations in Germanic and Balto-Slavic that are far more likely areal features than traceable to a common proto-language, such as the uniform development of a [[high vowel]] (*''u'' in the case of Germanic, *''i/u'' in the case of Baltic and Slavic) before the PIE syllabic resonants *''ṛ, *ḷ, *ṃ, *ṇ'', unique to these two groups among IE languages, which is in agreement with the wave model. The [[Balkan sprachbund]] even features areal convergence among members of very different branches.

An extension to the ''[[Donald Ringe|Ringe]]-[[Tandy Warnow|Warnow]] model of language evolution'' suggests that early IE had featured limited contact between distinct lineages, with only the Germanic subfamily exhibiting a less treelike behaviour as it acquired some characteristics from neighbours early in its evolution. The internal diversification of especially West Germanic is cited to have been radically non-treelike.<ref>{{Cite journal |last1=Nakhleh |first1=Luay |last2=Ringe |first2=Don |last3=Warnow |first3=Tandy |author3-link=Tandy Warnow |title=Perfect Phylogenetic Networks: A New Methodology for Reconstructing the Evolutionary History of Natural Languages |name-list-style=amp |date=2005 |journal=[[Language (journal)|Language]] |volume=81 |issue=2 |pages=382–420 |doi=10.1353/lan.2005.0078 |citeseerx=10.1.1.65.1791 |s2cid=162958 |url=http://www.cs.rice.edu/~nakhleh/Papers/NRWlanguage.pdf}}</ref>

=== Proposed subgroupings ===
{{Hypothetical Indo-European subfamilies}}
Specialists have postulated the existence of higher-order subgroups such as [[Italo-Celtic]], [[Graeco-Armenian]], [[Graeco-Aryan]] or Graeco-Armeno-Aryan, and Balto-Slavo-Germanic. However, unlike the ten traditional branches, these are all controversial to a greater or lesser degree.<ref>{{cite book |title=Encyclopedia of Indo-European Culture |publisher=Fitzroy Dearborn |last1=Mallory |first1=J. P. |last2=Adams |first2=D. Q. |year=1997 |location=London}}</ref>

The Italo-Celtic subgroup was at one point uncontroversial, considered by [[Antoine Meillet]] to be even better established than Balto-Slavic.{{sfn|Porzig|1954|p=39}} The main lines of evidence included the genitive suffix ''-ī''; the superlative suffix ''-m̥mo''; the change of /p/ to /kʷ/ before another /kʷ/ in the same word (as in ''penkʷe'' > ''*kʷenkʷe'' > Latin {{lang|la|quīnque}}, Old Irish {{lang|sga|cóic}}); and the subjunctive morpheme ''-ā-''.{{sfn|Fortson|2004|p=247}} This evidence was prominently challenged by [[Calvert Watkins]],<ref>{{cite encyclopedia |title=Italo-Celtic revisited |encyclopedia=Ancient Indo-European dialects |publisher=[[University of California Press]] |last=Watkins |first=Calvert |editor1-last=Birnbaum |editor1-first=Henrik |editor2-last=Puhvel |editor2-first=Jaan |year=1966 |location=Berkeley |pages=29–50}}</ref> while Michael Weiss has argued for the subgroup.<ref>{{cite conference |title=Italo-Celtica: linguistic and cultural points of contact between Italic and Celtic |conference=Proceedings of the 23rd annual UCLA Indo-European Conference |publisher=Hempen |last=Weiss |first=Michael |editor1-last=Jamison |editor1-first=Stephanie W. |editor2-last=Melchert |editor2-first=H. Craig |editor3-last=Vine |editor3-first=Brent |year=2012 |location=Bremen |pages=151–173 |url=https://www.academia.edu/3249855 |access-date=19 February 2018 |isbn=978-3-934106-99-4}}</ref>

Evidence for a relationship between Greek and Armenian includes the regular change of the [[Laryngeal theory|second laryngeal]] to ''a'' at the beginnings of words, as well as terms for "woman" and "sheep".<ref>{{cite journal |title=Review of ''The linguistic relationship between Armenian and Greek'' by James Clackson |last=Greppin |first=James |journal=[[Language (journal)|Language]] |year=1996 |volume=72 |issue=4 |pages=804–807 |doi=10.2307/416105 |jstor=416105}}</ref> Greek and Indo-Iranian share innovations mainly in verbal morphology and patterns of nominal derivation.<ref>{{cite book |title=Indoiranisch-griechische Gemeinsamkeiten der Nominalbildung und deren indogermanische Grundlagen |language=de |trans-title=Indo-Iranian-Greek similarities in nominal formation and their Indo-European foundations |publisher=Institut für Sprachwissenschaft der Universität Innsbruck |last=Euler |first=Wolfram |author-link=Wolfram Euler |year=1979 |location=Innsbruck}}</ref> Relations have also been proposed between Phrygian and Greek,{{sfn|Lubotsky|1988}} and between Thracian and Armenian.{{sfn|Kortlandt|1988}}<ref>{{cite book |last=Renfrew |first=Colin |author-link=Colin Renfrew |date=1987 |title=Archaeology & Language. The Puzzle of the Indo-European Origins |location=London |publisher=Jonathan Cape |isbn=978-0-224-02495-2}}</ref> Some fundamental shared features, like the [[aorist]] (a verb form denoting action without reference to duration or completion) having the perfect active particle -s fixed to the stem, link this group closer to Anatolian languages{{sfn|Encyclopædia Britannica|1981|p=593}} and Tocharian. Shared features with Balto-Slavic languages, on the other hand (especially present and preterit formations), might be due to later contacts.{{sfn|Encyclopædia Britannica|1981|loc=p. 667 George S. Lane, Douglas Q. Adams, ''The Tocharian problem''}}

The [[Indo-Hittite]] hypothesis proposes that the Indo-European language family consists of two main branches: one represented by the Anatolian languages and another branch encompassing all other Indo-European languages. Features that separate Anatolian from all other branches of Indo-European (such as the gender or the verb system) have been interpreted alternately as archaic debris or as innovations due to prolonged isolation. Points proffered in favour of the Indo-Hittite hypothesis are the (non-universal) Indo-European agricultural terminology in Anatolia<ref>The supposed autochthony of Hittites, the Indo-Hittite hypothesis and migration of agricultural "Indo-European" societies became intrinsically linked together by Colin Renfrew ({{harvnb|Renfrew|2001|pp=36–73}}).</ref> and the preservation of laryngeals.{{sfn|Encyclopædia Britannica|1981|loc=Houwink ten Cate, H. J.; Melchert, H. Craig & van den Hout, Theo P. J. p. 586 ''The parent language, Laryngeal theory''; pp. 589, 593 ''Anatolian languages''}} However, in general this hypothesis is considered to attribute too much weight to the Anatolian evidence. According to another view, the Anatolian subgroup left the Indo-European parent language comparatively late, approximately at the same time as Indo-Iranian and later than the Greek or Armenian divisions. A third view, especially prevalent in the so-called French school of Indo-European studies, holds that extant similarities in non-[[satem]] languages in general—including Anatolian—might be due to their peripheral location in the Indo-European language-area and to early separation, rather than indicating a special ancestral relationship.{{sfn|Encyclopædia Britannica|1981|loc=p. 594, ''Indo-Hittite hypothesis''}} Hans J. Holm, based on lexical calculations, arrives at a picture roughly replicating the general scholarly opinion and refuting the Indo-Hittite hypothesis.<ref>{{harvnb|Holm|2008|pp=629–636}}. The result is a partly new chain of separation for the main Indo-European branches, which fits well to the grammatical facts, as well as to the geographical distribution of these branches. In particular it clearly demonstrates that the Anatolian languages did not part as first ones and thereby refutes the Indo-Hittite hypothesis.</ref>

=== Satem and centum languages ===
{{Main|Centum and satem languages}}
[[File:Indo-European isoglosses.png|thumb|upright=1.6|Some significant isoglosses in Indo-European daughter languages at around 500&nbsp;BC.
{{Legend|#9fc7f3|Blue: centum languages}}
{{Legend|#ef7a6e|Red: satem languages}}
{{Legend|#f6a20f|Orange: languages with [[Augment (Indo-European)|augment]]}}
{{Legend|#a1f091|Green: languages with PIE *-tt- > -ss-}}
{{Legend|#f6d3ab|Tan: languages with PIE *-tt- > -st-}}
{{Legend|#fdd1d1|Pink: languages with instrumental, dative and ablative plural endings (and some others) in *-m- rather than *-bh-}}]]
The division of the Indo-European languages into satem and centum groups was put forward by Peter von Bradke in 1890, although [[Karl Brugmann]] did propose a similar type of division in 1886. In the satem languages, which include the Balto-Slavic and Indo-Iranian branches, as well as (in most respects) Albanian and Armenian, the reconstructed [[Proto-Indo-European phonology#Consonants|Proto-Indo-European palatovelars]] remained distinct and were fricativized, while the labiovelars merged with the 'plain velars'. In the centum languages, the palatovelars merged with the plain velars, while the labiovelars remained distinct. The results of these alternative developments are exemplified by the words for "hundred" in Avestan ({{lang|ae|satem}}) and Latin ({{lang|la|centum}})—the initial palatovelar developed into a fricative {{IPA|[s]}} in the former, but became an ordinary velar {{IPA|[k]}} in the latter.

Rather than being a genealogical separation, the centum–satem division is commonly seen as resulting from innovative changes that spread across PIE dialect-branches over a particular geographical area; the centum–satem [[isogloss]] intersects a number of other isoglosses that mark distinctions between features in the early IE branches. It may be that the centum branches in fact reflect the original state of affairs in PIE, and only the satem branches shared a set of innovations, which affected all but the peripheral areas of the PIE dialect continuum.{{sfn|Encyclopædia Britannica|1981|pp=588, 594}} Kortlandt proposes that the ancestors of Balts and Slavs took part in satemization before being drawn later into the western Indo-European sphere.{{sfn|Kortlandt|1990}}