In historical linguistics, Italo-Celtic is a grouping of the Italic and Celtic branches of the Indo-European language family on the basis of features shared by these two branches and no others. There is controversy about the causes of these similarities. They are usually considered to be innovations, likely to have developed after the breakup of the Proto-Indo-European language. It is also possible that some of these are not innovations, but shared conservative features, i.e. original Indo-European language features which have disappeared in all other language groups. What is commonly accepted is that the shared features may usefully be thought of as Italo-Celtic forms.
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Interpretations
The traditional interpretation of the data is that these two subgroups of the Indo-European language family are generally more closely related to each other than to the other Indo-European languages. This could imply that they are descended from a common ancestor, a Proto-Italo-Celtic which can be partly reconstructed by the comparative method. Those scholars who believe Proto-Italo-Celtic was an identifiable historical language estimate that it was spoken in the third or second millennium BC somewhere in south-central Europe but this hypothesis fell out of favour after being reexamined by Calvert Watkins in 1966. Some scholars, such as Frederik Kortlandt, continued to be interested in the theory. In 2002 a paper by Ringe, Warnow, and Taylor, employing computational methods as a supplement to the traditional linguistic subgrouping methodology, argued in favour of an Italo-Celtic subgroup, and in 2007 Kortlandt attempted a reconstruction of a Proto-Italo-Celtic.
The most common alternative interpretation is that the close proximity of Proto-Celtic and Proto-Italic over a long period could have encouraged the parallel development of what were already quite separate languages; areal features within a Sprachbund. As Watkins (1966) puts it, "the community of -ī in Italic and Celtic is attributable to early contact, rather than to an original unity." The assumed period of language contact could then be later, perhaps continuing well into the first millennium BC.
However, if some of the forms are archaic elements of Proto-Indo-European that were lost in other branches, neither model of post-PIE relationship need be postulated. Italic and especially Celtic also share some distinctive features with the Hittite language (an Anatolian language) and the Tocharian languages, and these features are certainly archaisms.
Forms
The principal Italo-Celtic forms are:
Other similarities include the fact that certain common words, such as the words for common metals (gold, silver, tin, etc.) are similar in Italic and Celtic yet divergent from other Indo-European languages. A number of other similarities continue to be pointed out and debated.
The r-passive (mediopassive voice) was initially thought to be an innovation restricted to Italo-Celtic until it was found to be a retained archaism shared with Hittite, Tocharian, and possibly the Phrygian language.