The Centum-Satem division is an isogloss 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:
The terms Centum Group and Satem Group come from the words for the number "one hundred" in a traditional representative language of each group: Latin centum and Avestan satəm. The initial consonant in these two examples comes from the Indo-European "palatovelar" consonant, *ḱ, which became in the first case a simple velar, and in the second a sibilant.
The terms "palatovelar" and "plain velar" are in quotes because they are traditional terms but do not reflect current thinking, which holds that the "palatovelars" were actually plain velars, e.g. [k], while the "plain velars" were pronounced farther back in the mouth, perhaps as uvular consonants (e.g. [q]).
The Satem languages (which have the sibilant where the centum equivalents have the velar) include Indo-Iranian, Armenian, Balto-Slavic, Albanian, and perhaps also a number of barely documented extinct languages, such as Thracian-Dacian. This group changed PIE palatovelars into sibilants, retaining PIE plain velars and merging PIE labiovelars into them to form an expanded velar group. It is sometimes suggested that the plain velars and the labiovelars were not merged in Proto-Albanian, but this is not a mainstream viewpoint. Balto-Slavic is largely Satem but evidences Centum development in some words, suggesting that "Satemization" was incomplete or operated according to different principles than in the other Satem languages.
The Centum group includes all remaining dialects, i.e. Italic, Celtic, Germanic, Hellenic, Tocharian, Anatolian and possibly a number of lesser-known extinct groups (such as Ancient Macedonian and Venetic). This group merged PIE palatovelars and plain velars, yielding plain velars (but see below about Anatolian).
The Satem languages share some innovations in common (particularly, the ruki sound law), while the Centum languages have no common innovations, and in fact include the two groups that split off the earliest, i.e. Anatolian and Tocharian. Furthermore, the Satem languages occur in a contiguous region approximately in the middle of the PIE area, while the Centum languages occur in a discontiguous area partly surrounding the Satem languages. This strongly suggests that "Satemization" was a single areal sound change, which occurred less completely in Balto-Slavic (at the edge of the area) than elsewhere, while "Centumization" was actually a set of unrelated changes occurring independently in multiple language groups. This is easy to understand given the current conception of the PIE values of the three dorsal series, where Centumization involves nothing more than the elimination of the velar-uvular distinction, which is typologically fairly rare and in any case carried very little functional load in PIE. Satemization, however, was a more substantial change, involving the fronting of PIE velars and the unrounding of PIE labiovelars.
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