A proto-language, in the tree model of historical linguistics, is a language, usually hypothetical or reconstructed, and usually unattested, from which a number of attested known languages are believed to have descended by evolution, forming a language family.
In the strict sense, a proto-language is the most recent common ancestor of a language family, immediately before the family started to diverge into the attested daughter languages. It is therefore equivalent with the ancestral language or parental language of a language family.
Moreover, a group of idioms (such as a dialect cluster) which are not considered separate languages (for whichever reasons) can also be described as descending from a unitary proto-language.
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 labelled "compatible".
No trees but the smallest branches are ever found to be perfect, in part because languages also evolve through horizontal transfer with their neighbours. 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 (and probably ultimately inspired it), a literary history exists from as early as a few millennia ago, allowing the descent to be traced in detail. The early daughter languages, and even the proto-language itself, may be 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.
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 to 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.
There are no objective criteria for the evaluation of different reconstruction systems yielding different proto-languages. Many researchers concerned with linguistic reconstruction agree that the traditional comparative method is an "intuitive undertaking".
The bias of the researchers regarding the accumulated implicit knowledge can also lead to erroneous assumptions and their generalization. Kortlandt (1993) lists several cases where such general assumptions concerning "the nature of language" turned out to hinder the research in the field of historical linguistics. Linguists make their judgements on what they consider "natural" for a language to change, and "As a result, our reconstructions tend to have a strong bias toward the average language type known to the investigator."
The advent of wave model raised new issues in the domain of linguistic reconstruction, causing the reevaluation of old reconstruction systems and depriving the proto-language of its "uniform character". This is evident in Karl Brugmann's skepticism that the reconstruction systems could ever reflect a linguistic reality. Ferdinand de Saussure went further, completely rejecting a positive specification of the sound values of reconstruction systems.
In general, the issue of the nature of proto-language remains unsolved, with linguists taking the realist or abstractionist position. Even the widely studied proto-languages, such as Proto-Indo-European, have suffered criticism due to being typological outliers with respect to the reconstructed phonemic inventory. The alternatives such as glottalic theory, despite representing a typologically less rare system, have not gained wider acceptance, with some researchers even suggesting the use of indexes to represent the disputed series of plosives. On the other end of spectrum, Pulgram (1959:424) suggests that Proto-Indo-European reconstructions are just "a set of reconstructed formulae not representative of any reality". In the same vein Julius Pokorny in his study on Indo-European claims that the linguistic term IE parent language is merely an abstraction that does not exist in reality, and it should be understood as consisting of dialects possibly dating back to the paleolithic, during which these formed the linguistic structure of the IE language group. In his view, Indo-European is solely a system of isoglosses which held together dialects which were spoken by various tribes, from which the historically attested Indo-European languages emerged.
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.