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A proto-language (alternatively known as a parent language, or common ancestor) in the tree model of historical linguistics is a hypothetical, or reconstructed, typically extinct language from which a number of attested, or documented, known languages are believed to have descended by evolution, or slow modification of the proto-language into languages that form a language family. Occasionally, the German term Ursprache (from Ur- "primordial" and Sprache "language", pronounced [ˈʔuːɐ.ʃpʁaː.xə]) is used instead.
Definition and verification of proto-languages 
Typically, the proto-language is not known directly. It is by definition a linguistic reconstruction formulated by applying the comparative method to a group of languages featuring similar characteristics. The tree is a statement of similarity and a hypothesis that the similarity results from descent from a common language.
The comparative method, a process of deduction, begins from a set of characteristics, or characters, found in the attested languages. If the entire set can be accounted for by descent from the proto-language, which must contain the proto-forms of them all, the tree, or phylogeny, is regarded as a complete explanation and by Occam's razor, is given credibility. More recently such a tree has been termed "perfect" and the characters labeled "compatible."
No trees but the smallest branches are ever found to be perfect. Typically credibility is given to the hypotheses of highest compatibility. The differences in compatibility must be explained by various applications of the wave model. The level of completeness of the reconstruction achieved varies, depending on how complete the evidence is from the descendant languages and on the formulation of the characters by the linguists working on it. Not all characters are suitable for the comparative method. For example, lexical items that are loans from a different language do not reflect the phylogeny to be tested, and if used will detract from the compatibility. Getting the right dataset for the comparative method is a major task in historical linguistics.
In a few fortuitous instances, which have been used to verify the method and the model, a literary history exists from as early as a few millennia ago. The descent can be traced in detail. The daughter languages are attested in surviving texts. For example, Latin is the proto-language of the Romance language family, which includes such modern languages as French, Italian, Portuguese, Romanian, Catalan and Spanish. Likewise, Proto-Norse, the ancestor of the modern Scandinavian languages, is attested, albeit in fragmentary form, in the Elder Futhark. Although there are no very early Indo-Aryan inscriptions, the Indo-Aryan languages of modern India all go back to Vedic Sanskrit (or dialects very closely related to it), which has been preserved in texts accurately handed down by parallel oral and written traditions for many centuries.
Proto-X vs. Pre-X 
Normally, the term "Proto-X" refers to the last common ancestor of a group of languages, occasionally attested but most commonly reconstructed through the comparative method. An earlier stage of the same language, reconstructed through the method of internal reconstruction, is termed "Pre-X". This terminology is used, for example, in the case of Proto-Indo-European and Pre-Indo-European; likewise for Proto-Germanic and Pre-Germanic. As Pre-X is sometimes also used for a postulated substratum (for example Pre-Germanic can also refer a hypothetical Germanic substratum), the more precise term Pre-Proto-X is sometimes used, for a stage older than the last common ancestor of the attested language varieties employed in the reconstruction.
When multiple historical stages of a single language exist, the oldest attested stage is normally termed "Old X" (e.g. "Old English", "Old Korean"). For an earlier, hypothetical stage, reconstructed through the method of internal reconstruction, terminology differs, with some authors using "Proto-X" (e.g. "Proto-English") and others using "Pre-X" (e.g. "Pre-English"). The case of Irish is somewhat different; the language known as Old Irish is the language in which the first significant texts are known, but an older stage named Primitive Irish is also attested, though much more sparsely. This is similar to the situation of Old Norse and Proto-Norse, where both are attested but the latter only fragmentarily.
See also 
- List of proto-languages
- Comparative method
- Internal reconstruction
- Historical linguistics
- Proto-Human language
- Universal language
- Koerner, E F K (1999). Linguistic historiography: projects & prospects. Amsterdam studies in the theory and history of linguistic science; Ser. 3, Studies in the history of the language sciences. Amsterdam [u.a.]: J. Benjamins. p. 109. "First, the historical linguist does not reconstruct a language (or part of the language) but a model which represents or is intended to represent the underlying system or systems of such a language."
- Lehmann 1993, p. 26.
||This article includes a list of references, but its sources remain unclear because it has insufficient inline citations. (June 2011)|
- Lehmann, Winfred P. (1993). Theoretical Bases of Indo-European Linguistics. London, New York: Taylor & Francis Group (Routledge).
- Schleicher, August (1861–1862). Compendium der vergleichenden Grammatik der indogermanischen Sprachen: 2 volumes. Weimar: H. Boehlau (Reprint: Minerva GmbH, Wissenschaftlicher Verlag). ISBN 3-8102-1071-4.