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Similar developments appear in Italic, but for the syllabic nasals *m̥, *n̥, the result is Proto-Italic *əm, *ən (> Latin em ~ im, en ~ in). One change shows non-exact parallels in Italic: vocalization of syllabic resonants next to laryngeals depending on the environment. *p assimilates to *kʷ when another *kʷ follows later in the word (p…kʷ > kʷ…kʷ).
*m is assimilated or lost before a glide:. Syllabic resonants before a voiced unaspirated stop become *Ra (R̥D > RaD). Syllabic laryngeals become *a (CHC > CaC).
Possibly, vocalization of laryngeals to *ī between a *CR cluster and consonantal *j (CRHjV > CRījV). Development of initial stress, following the previous two changes. Possibly, post-consonantal laryngeals are lost when before pre-tonic close vowels:. Dybo's rule: long close vowels are shortened (or a laryngeal is lost) before resonant + stressed vowel. The following sound changes are shared with the Italic languages in particular, and are cited in support of the Italo-Celtic hypothesis. Two adjacent dentals become two adjacent sibilants (TT > ss). between plosives in non-initial syllables (CHC > CC). following a vowel, resulting in compensatory lengthening, thus (VH > V̄). following a vowel in syllables before the accent (VHC´ > VC´). Epenthetic *a is inserted after a syllabic sonorant if a laryngeal and another sonorant follow (R̥HR > RaHR). Palatovelars merge into the plain velars:. *e is colored by an adjacent laryngeal consonant:. These changes are shared by several other Indo-European branches. The changes are roughly in chronological order, with changes that operate on the outcome of earlier ones appearing later in the list. The phonological changes from Proto-Indo-European (PIE) to Proto-Celtic (PC) may be summarized as follows. The Proto-Celtic word for 'iron', traditionally reconstructed as *īsarnom, in particular, has long been taken as an indication that the divergence into individual Celtic languages did not start until the Iron Age (8th century BC at the latest), but Schumacher and Schrijver propose to date Proto-Celtic as early as the 13th century BC, the time of the Canegrate culture, in northwest Italy, and the Urnfield culture in Central Europe, implying that the divergence may have already started in the Bronze Age. Proto-Celtic is mostly dated to the Late Bronze Age, ca. 2 Sound changes from Proto-Indo-European. So the main sources for reconstruction come from Insular Celtic languages with the oldest literature found in Old Irish and Middle Welsh, dating back to authors flourishing in the 6th century AD. Though Continental Celtic presents much substantiation for Proto-Celtic phonology, and some for its morphology, recorded material is too scanty to allow a secure reconstruction of syntax, though some complete sentences are recorded in the Continental Gaulish and Celtiberian.
Proto-Celtic is currently being reconstructed, by necessity relying on later Celtic languages. Celtic languages share common features with Italic languages that are not found in other branches of Indo-European, suggesting the possibility of an earlier Italo-Celtic linguistic unity. Proto-Celtic is usually associated with the Urnfield or Hallstatt cultures. Proto-Celtic is generally thought to have been spoken between 1300 and 800 BC, after which it began to split into different languages. It is not attested in writing, but has been partly reconstructed through the comparative method.
Proto-Celtic, or Common Celtic, is the ancestral proto-language of all known Celtic languages, and a descendant of Proto-Indo-European.