In historical linguistics, internal reconstruction is a method of reconstructing an earlier state in a language's history using only evidence internal to the language in question. The comparative method compares variations between languages, such as in sets of cognates, under the assumption that they descend from a single proto-language, but internal reconstruction compares variant forms within a single language under the assumption that they descend from a single, regular form. For example, they could take the form of allomorphs of the same morpheme. The basic premise of internal reconstruction is that a meaning-bearing element that alternates between two or more similar forms in different environments was probably once a single form into which alternation has been introduced by the usual mechanisms of sound change and analogy. Language forms that are reconstructed by internal reconstruction are denoted with the pre- prefix, as in Pre-Old Japanese, like the use of proto- to indicate a language reconstructed by means of the comparative method, as in Proto-Indo-European. (However, the pre- prefix is sometimes used for an unattested prior stage of a language, without reference to internal reconstruction.) It is possible to apply internal reconstruction even to proto-languages reconstructed by the comparative method. For example, performing internal reconstruction on Proto-Mayan would yield Pre-Proto-Mayan. In some cases, it is also desirable to use internal reconstruction to uncover an earlier form of various languages and then submit those pre- languages to the comparative method. Care must be taken, however, because internal reconstruction performed on languages before the comparative method is applied can remove significant evidence of the earlier state of the language and thus reduce the accuracy of the reconstructed proto-language.
Role in historical linguistics When undertaking a comparative study of an underanalyzed language family, one should understand its systems of alternations, if any, before one tackles the greater complexities of analyzing entire linguistic structures. For example, Type A forms of verbs in Samoan are the citation forms, which are in dictionaries and word lists, but in making historical comparisons with other Austronesian languages, one should not use Samoan citation forms that have missing parts. (An analysis of the verb sets would alert the researcher to the certainty that many other words in Samoan have lost a final consonant.) In other words, internal reconstruction gives access to an earlier stage, at least in some details, of the languages being compared, which can be valuable since the more time has passed, the more changes have been accumulated in the structure of a living language. Thus, the earliest known attestations of languages should be used with the comparative method. Internal reconstruction, when it is not a sort of preliminary to the application of the comparative method, is most useful if the analytic power of the comparative method is unavailable, especially in language isolates. Internal reconstruction can also draw limited inferences from peculiarities of distribution. Even before comparative investigations had sorted out the true history of Indo-Iranian phonology, some scholars had wondered if the extraordinary frequency of the phoneme /a/ in Sanskrit – 20% of all phonemes together – might point to some historical fusion of two or more vowels. (In fact, it represents the final outcome of five different Proto-Indo-European syllabics whose syllabic states of /m/ and /n/ can be discerned by the application of internal reconstruction.) However, in such cases, internal analysis is better at raising questions than at answering them. The extraordinary frequency of /a/ in Sanskrit hints at some sort of historical event but does not and cannot lead to any specific theory.
Issues and shortcomings
Neutralizing environments One issue in internal reconstruction is neutralizing environments, which can be an obstacle to historically correct analysis. Consider the following forms from Spanish, spelled phonemically rather than orthographically:
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