The centum–satem division is one of many isoglosses of the Indo-European language family, related to the different evolution of the three dorsal consonant rows of the mainstream reconstruction of Proto-Indo-European (PIE):
The centum group includes Italic, Celtic, Germanic, Hellenic and Tocharian. This group merged PIE palatovelars and plain velars yielding plain velars only, but retain the labiovelars as a distinct set. Tocharian largely reflects a situation where all three PIE dorsal series as well as all voicing/aspiration distinctions (originally constituting nine separate consonants) have merged into a single phoneme /k/. This has led some writers to suggest that Tocharian does not fit the Centum–Satem model. However, some PIE labiovelars are in fact represented by a labiovelar-like element or by a non-original sequence /ku/. Along with other evidence, this suggests that labiovelars were distinct in Proto-Tocharian and only later merged with velars (as happened independently in Old Irish and to some extent in some other languages), making Tocharian a clearly Centum language.
The satem languages (which have the sibilant where the centum equivalents have the velar) include Baltic, Slavic, Armenian and Indo-Iranian. This group lose the labial element of the PIE labiovelars and thus merge them with the dorso-velars while the dorso-palatals remain distinct. Balto-Slavic is largely satem but evidences centum development in some words, suggesting that "Satemization" was incomplete. There is residual evidence of various sorts in satem languages of a former distinction between velar and labiovelar consonants.
Recent evidence from Luwian indicates that all three dorsal consonant rows were maintained separately in Proto-Anatolian, and the Centumization observed in Hittite only occurred after the breakup of Common Anatolian.
The isogloss only applies to the parent language with the full inventory of dorsals. Later sound changes within a specific branch of Indo-European that are analogous to one of the centum or satem changes, such as the palatalization of Latin k to s in some Romance languages or the merger of *kʷ with *k in the Goidelic languages, are excluded.
Proto-Indo-European dorsals 
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History of the concept 
Schleicher's single guttural row 
August Schleicher, an early Indo-Europeanist, in Part I, "Phonology", of his major work, the 1871 "Compendium of Comparative Grammar of the Indogermanic Language", published a table of original momentane Laute, or "stops", that has only a single velar row, *k, *g, *gʰ, under the name of Gutturalen. He does identify four palatals (*ḱ, *ǵ, *ḱʰ, *ǵʰ) but hypothesizes that they came from the gutturals along with the nasal ń and the spirant ç.
Brugmann's labialized and unlabialized language groups 
Karl Brugmann in his 1886 equivalent work, "Outline of Comparative Grammar of the Indogermanic Language," promotes the palatals to the original language, recognizing two rows of Explosivae, or "stops", the palatal (*ḱ, *ǵ, *ḱʰ, *ǵʰ) and the velar (*k, *g, *kʰ, *gʰ), each of which was simplified to three articulations even in the same work. In that same work Brugmann notices among die velaren Verschlusslaute, "the velar stops", a major contrast between reflexes of the same words in different daughter languages: in some the velar is marked with a u-Sprache, "u-articulation," which he terms a Labialisierung, "labialization," in accordance with the prevailing theory that the labiovelars were velars labialized by combination with a u at some later time and not among the original consonants. He divides languages therefore into die Sprachgruppe mit Labialisierung and die Sprachgruppe ohne Labialisierung, "the language group with (or without) labialization," which are perforce identical to the Centum and Satem groups. He opines that
"For words and groups of words, which do not appear in any language with labialized velar-sound [the "pure velars"], it must for the present be left undecided whether they ever had the u-afterclap."
The doubt introduced in this passage suggests he already suspected the "afterclap" u was not that but was part of an original sound.
Von Bradke's centum and satem groups 
In 1890 Peter von Bradke published "Concerning Method and Conclusions of Aryan (Indogermanic) Studies" in which he saw the same division (Trennung) as did Brugmann but he defined it in a different way. He said that the original Aryans knew two kinds of gutturaler Laute, or "guttural sounds," the gutturale oder velare, und die palatale Reihe, "guttural or velar and palatal rows," each of which were aspirated and unaspirated. The velars were to be viewed as gutturals in an engerer Sinn, "narrow sense." They were a reiner K-Laut, "pure K-sound." Palatals were häufig mit nachfolgender Labialisierung, "frequently with subsequent Labialization." This latter distinction led him to divide the palatale Reihe into a Gruppe als Spirant and a reiner K-Laut, typified by the words satem and centum respectively. Later in the book he speaks of an original centum-Gruppe from which on the north of the Black and Caspian Seas the satem-Stämmen dissimilated among the Nomadenvölker, or Steppenvölker, located there by further palatalization of the palatal gutturals.
Brugmann's identification of labialized and centum 
By the 1897 edition of Grundriss, Brugmann (and Delbrück) had adopted Von Bradke's view. He says
"Die Palatallaute der idg. Urzeit ... erscheinen in Griech, Ital., Kelt., Germ. in der Regel als K-Laute, dagegen im Ar., Arm., Alb., Balt-Slav., denen sich Phrygisch und Thrakisch ... meistens als Zischlaute."[The Proto-Indo-European palatals appear in Greek, Italic, Celtic and Germanic as a rule as K-sounds, as opposed to in Aryan, Armenian, Albanian, Balto-Slavic, Phrygian and Thracian for the most part sibilants.]
Concerning the labialized velars Brugmann had changed his mind, and there was no more mention of labialized and non-labialized language groups. The labio-velars now appeared under that name as one row of the 5-row Verschlusslaute (Explosivae) containing die labialen V., die dentalen V., die palatalen V., die reinvelaren V. and die labiovelaren V. It was Brugmann who pointed out that labiovelars had merged into the velars in the Satem Group, accounting for the coincidence of the discarded non-labialized group with the Satem Group.
The Satem concept 
The Satem languages show characteristic affricate and fricative consonants articulated in the front of the mouth in inherited Indo-European lexical items in which in other languages termed the Centum Languages pure velars and labiovelars, sounds articulated at the back of the mouth, are found. The Satem shift is conveniently illustrated with the word for '100', Proto-Indo-European *(d)ḱm̥tóm, which became Avestan satəm (hence the name of the group), Persian sad, Sanskrit śatam, Latvian simts, Lithuanian šimtas, Old Church Slavonic sъto. Another example is the Slavic prefix sъ(n)- ("with"), which appears in Latin, a centum language, as co(n)-; conjoin is cognate with Russian soyuz ("union").
The sources of the satem sounds and the methods by which they became what they are have been debated heavily by Indo-European linguists for many decades. The originator of the concept, Peter von Bradke, believed in a Proto-Indo-European two-row system of four gutturals each row, the pure velar row: *k, *kʰ, *g, *gʰ, and the palatovelar row: *ḱ, *ḱʰ, *ǵ, *ǵʰ. For example, *ḱ became Sanskrit ś [ɕ], Latvian, Avestan, Russian and Armenian s, Lithuanian š [ʃ], and Albanian th [θ] (but k before a resonant). Karl Brugmann added the labio-velar row: *kʷ, *kʷʰ, *gʷ, *gʷʰ, with the proviso that in the Satem languages it merged into the velar row, losing their accompanying lip-rounding. This merger left the Satem group without labio-velars. Regardless of whether satem words were created from those rows with those articulations in that way, they are definable as satem words.
Satem-like features have arisen multiple times during history (e.g. French cent pron. [sã]), Romanian Banat dialect ʃinʃ /ṡinṡ ”five” ; Moldavian dialect. As a result, it is sometimes difficult to firmly establish which languages were part of the original Satem diffusion and which were affected by secondary assibilation in a later time period. For instance, it is known that the assibilation found in French and Swedish were later developments as linguists have extensive documentation of Latin and Old Swedish. However, in the case of Dacian and Thracian, there is not enough information on the history of these languages to conclusively settle the issue of when their Satem-like features originated. Extensive lexical borrowing, such as Armenian from Iranian, may also add to the difficulty. The status of Armenian as a Satem language as opposed to a Centum language with secondary assibilation rests on the evidence of a very few words.
The Centum concept 
The Centum languages show characteristic pure velars and labiovelars articulated at the back of the mouth in inherited Indo-European lexical items in which in other languages termed the Satem Languages affricate and fricative consonants articulated in the front of the mouth are found. The name Centum comes from the Latin word centum (pronounced [kentum]) < PIE *ḱm̥tóm, '100', English hund(red)- (with /h/ from earlier *k, see Grimm's law), Greek (he)katon, Welsh cant, Tocharian B kante. Labiovelars as single phonemes (for example, /kʷ/), as opposed to biphonemes (for example, /kw/) are attested in Greek (the Linear B q- series), Italic (Latin qu), Germanic (Gothic hwair ƕ and qairþra q) and Celtic (Ogham ceirt Q). In the Centum languages, the palatovelar consonants merged into the plain velars (*k, *g, *gʰ). The merger left the Centum Group without palatovelars.
The Centum languages preserve Proto-Indo-European labiovelars (*kʷ, *gʷ, *gʷʰ) or their historical reflexes as distinct from plain velars; for example, PIE *k : *kʷ > Latin c /k/ : qu /kʷ/, Greek κ /k/ : π /p/ (or τ /t/ before front vowels), Gothic /h/ : /hʷ/, etc. Remnants of labial elements from labiovelars in Balto-Slavic include Lithuanian ungurys "eel" < *angʷi- , Lithuanian dygus "pointy" < *dʰeigʷ-. Fewer examples of incomplete Satemization are also known from Indo-Iranian, such as Sanskrit guru "heavy" < *gʷer-, kulam "herd" < *kʷel-; kuru "make" < *kʷer- may be compared, but they arise only post-Rigvedic in attested texts.
Historical interpretation of the sound changes 
When von Bradke first published his definition of the Centum and Satem Languages, he viewed his classification as "the oldest perceivable division" in Indo-European, which he elucidated as "a division between eastern and western cultural provinces (Kulturkreises)." This proposed split was undermined by the discoveries of Hittite and Tocharian, which were Centum languages located within the hypothetical Satem range, Tocharian isolated on the Silk Route in the far east, divided from its closest cognates in Europe by thousands of miles of rugged terrain and hostile peoples. This proposed first division based on a single isogloss was further weakened by continued research into additional Indo-European isoglosses, many of which seemed of equal or greater importance in the development of daughter languages. Philip Baldi explains:
"...an early dialect split of the type indicated by the centum-satem contrast should be expected to be reflected in other high-order dialect distinctions as well, a pattern which is not evident from an analysis of shared features among eastern and western languages."
To this claim one could oppose the example of Old High German and Old Saxon, which, although separated by a drastic consonant shift, stayed almost identical in grammar and syntax for several centuries.
The Centum–Satem isogloss is now understood to be a chronological development of PIE. Centumization removed the palatovelars from the language, leaving none to satemize. In addition there is residual evidence of various sorts in satem languages of a former distinction between velar and labiovelar consonants, indicating the earlier centum state. It is therefore clear that centumization was followed by satemization. However the evidence of Anatolian indicates that centum was not the original state of PIE.
Alternative views 
The presence of three dorsal rows in the proto-language is the mainstream hypothesis. In another hypothesis by Antoine Meillet the original rows were the labiovelars and palatovelars, with the pure velars being allophones of the palatovelars in some cases, such as depalatalization before a resonant. Other possibilities are borrowing between early daughter languages during the process of Satemization, or perhaps the concept of original velars is an artifact based on just plain false etymologies in modern times. Oswald Szemerényi proposed that the "preconsonantal palatals probably owe their origin, at least in part, to a lost palatal vowel;" that is, a velar was palatalized by a following vowel subsequently lost. The palatal row therefore post-dated the original velar and labiovelar but Szemerényi does not give times. He includes the palatals in a table of five rows of stops "shortly before the break-up" with a question mark after them. Other scholars who assume two dorsal rows in PIE include Kuryłowicz (1935), Meillet (1937), Lehmann (1952), and Woodhouse (1998).
See also 
- J.P. Mallory and D.Q. Adams (eds.), The Encyclopedia of Indo-European Culture (1997), p. 461.
- Lyovin 1997, p. 53
- Fortson 2010, p. 59
- Mallory 1997, p. 461.
- Fortson 2010, p. 59, originally proposed in Melchert 1987
- Fortson 2010, p. 178
- Schleicher 1871, p. 10
- Bynon, Theodora, "The Synthesis of Comparative and Historical Indo-European Studies: August Schleicher", in Auroux, Sylvain, History of the language sciences: an international handbook on, Volume 2, pp. 1223–1239
- Schleicher 1871, p. 163
- Brugmann 1886, p. 20
- Brugmann 1886, pp. 308–309
- Brugmann 1886, p. 312
- Brugmann 1886, p. 313. The quote given here is a translation by Joseph Wright, 1888.
- von Bradke 1890, p. 63
- von Bradke 1890, p. 107
- Brugmann & Delbrück 1897 p. 542.
- Brugmann & Delbrück 1897 p. 616. "...die Vertretung der qʷ-Laute ... ist wie die der q-Laute, ...."
- von Bradke 1890, p. 108
- K Shields, A New Look at the Centum/Satem Isogloss, Zeitschrift für vergleichende Sprachforschung (1981).
- Baldi, Philip (1999). The Foundations of Latin. Trends in Linguistics: Studies and Monographs 117. Berlin: Walter de Gruyter & Co. p. 39. ISBN 978-3-11-016294-3.
- Lehmann 1993, p. 100
- Szemerényi 1990, p. 148
- Brugmann, Karl (1886). Grundriss der Vergleichenden Grammatik der indogermanischen Sprachen (in German). Erster Band. Strassburg: Karl J. Trübner.
- Brugmann, Karl; Delbrück, Berthold (1897-1916). Grundriss der vergleichenden grammatik der indogermanischen sprachen. Volume I Part 1 (2nd ed.). Strassburg: K.J. Trübner.
- Fortson, Benjamin W. (2010). Indo-European Language and Culture: An Introduction. Blackwell Textbooks in Linguistics (2nd ed.). Chichester, U.K.; Malden, MA: Wiley-Blackwell.
- Kortlandt, Frederik (1993). "General Linguistics & Indo-European Reconstruction" (PDF). Frederik Kortlandt. Retrieved 30 November 2009.
- Lehmann, Winfred Philipp (1993). Theoretical Bases of Indo-European Linguistics. Taylor & Francis Group.
- Lyovin, Anatole (1997). An introduction to the languages of the world. New York: Oxford University Press.
- Mallory, J.P.; Adams, D.Q., eds. (1997). "Proto-Indo-European". Encyclopedia of Indo-European Culture. London, Chicago: Fitzroy Dearborn Publishers. ISBN 1-884964-98-2.
- Melchert, Craig (1987), "PIE velars in Luvian" (PDF), Studies in Memory of Warren Cowgill: 182–204, retrieved 28 November 2009.
- Remys, Edmund (2007). "General distinguishing features of various Indo-European languages and their relationship to Lithuanian". Indogermanische Forschungen (IF) 112: 244–276.
- Schleicher, August (1871). Compendium der vergleichenden grammatik der indogermanischen sprachen (in German). Weimar: Hermann Böhlau.
- Solta, G.R. (1965). "Palatalisierung und Labialisierung". Indogermanische Forschungen (IF) (in German) 70: 276–315.
- Szemerényi, Oswald J. L. (1990). Introduction to Indo-European Linguistics. Oxford [u.a.]: Oxford University Press.
- von Bradke, Peter (1890). Über Methode und Ergebnisse der arischen (indogermanischen) Alterthumswissenshaft (in German). Giessen: J. Ricker'che Buchhandlung.
- Allen, James. "Inferring Prehistory from Language Genealogy". James Allen. Retrieved 5 December 2009.
- Anderson, Deborah. "Centum and satem Languages". EurAsia '98. Retrieved 5 December 2009.
- Baldia, Maximilian O. (2006). "Tombs and the Indo-European Homeland". The Comparative Archaeology Web. Retrieved 27 November 2009.
- Wheeler, L. Kip. "The Indo-European Family of Languages". L. Kip Wheeler. Retrieved 5 December 2009.