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Internal reconstruction is a method of recovering information about a language's past from the characteristics of the language at a later date.
Whereas the comparative method compares variations between languages, such as in sets of cognates, under the assumption that they descend from a single proto-language, 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 a single form in the past into which alternation was introduced by the usual mechanisms of sound change and analogy. 
Language forms reconstructed by means of 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 even to apply internal reconstruction 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 applying the comparative method can remove significant evidence of the earlier state of the language and so reduce the accuracy of the reconstructed proto-language.
When undertaking a comparative study of a hitherto under analyzed family of languages it is worthwhile to get an understanding of their systems of alternations, if any, before tackling the greater complexities of analyzing entire linguistic structures. For example, Type A forms of verbs in Samoan (as in the example, below) are the citation forms, the forms in dictionaries and word lists, but when making historical comparisons with other Austronesian languages, one should not use Samoan citation forms with parts missing. (An analysis of the verb sets would alert the researcher to the certainty that many other words in Samoan have lost a final consonant.) Another way of looking at it is that internal reconstruction gives access to an earlier historical stage, at least in some details, of the languages being compared, which can be valuable: the more time that passes, the more changes accumulate in the structure of a (living) languages. Thus, the earliest known attestations of languages should be used with the comparative method.[ citation needed]
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.[ citation needed]
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, an astonishing total) 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 of which the 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.
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:
|Infinitive||Third person singular|
One pattern of inflection shows alternation between /o/ and /ue/; the other type has /o/ throughout. The lexical items are all basic, not technical or high register or obvious borrowings, so their behavior is likely to be a matter of inheritance from an earlier system rather than the result of some native pattern overlaid by a borrowed one. (An example of such an overlay would be the non-alternating English privative prefix un- next to the privative prefix in borrowed Latinate forms, which alternates between in-, im, ir-, il-.)
One might guess that the difference between the two sets can be explained by two aboriginally different markers of the third person singular, but a basic principle of linguistic analysis is that one cannot analyze data one does not have (and should not try to). Besides that, positing such a history violates the principle of parsimony ( Occam's Razor): it adds a complication to the analysis unnecessarily, a complication whose chief result is to restate the observed data as a sort of historical fact. That is, the result of the analysis is the same as the input. As it happens, the forms as given yield readily to real analysis, so there is no reason to look elsewhere.
The first assumption is that in pairs like bolbér/buélbe, the root vowels were originally the same. There are two possibilities: either something happened to make an original */o/ turn into two different sounds in the third person singular or the distinction in the third singular is original, and the vowels of the infinitives are in what is called a neutralizing environment (if an original contrast is lost because two or more elements "fall together", or coalesce into one). There is no way of telling ("predicting") when /o/ will break to /ué/ and when it will remain /ó/ in the third person singular. On the other hand, starting with /ó/ and /ué/, one can write an unambiguous rule for the infinitive forms: /ué/ becomes /o/. One might notice further, upon looking around in Spanish, that the nucleus /ue/ is found only in tonic syllables anywhere, not just in verb forms.
That analysis gains plausibility from the observation that the neutralizing environment is atonic, but the nuclei are different in tonic syllables. That fits with the commonplace that vowel contrasts are often preserved differently in tonic and atonic environments and that the usual relationship is that there are more contrasts in tonic syllables than in atonic ones (as previously distinctive vowels had fallen together in the atonic environment).
The idea that original */ue/ might fall together with original */o/ is unproblematic, so internally, a complex nucleus *ue can be reconstructed that remains distinct when tonic and coalesces with *o when atonic.
However, the true history is quite different: there were no diphthongs in Proto-Romance. There was an *o (reflecting Latin ŭ and ō) and an *ɔ (reflecting Latin ŏ). In Spanish, as in most other Romance languages, the two fall together in atonic syllables, but in stressed syllables, the *ɔ breaks into the complex nucleus /ue/. Internal reconstruction accurately points to two different historical nuclei in atonic /o/, but gets the details wrong.
When applying internal reconstruction to related languages prior to applying the comparative method, one must check that the analysis does not remove the shared innovations that characterize subgroups. An example is consonant gradation in Finnish, Estonian, and Sami. A pre-gradation phonology can be derived for each of the three groups via internal reconstruction, but in fact, it was manifestly an innovation in the Finnic branch of Uralic rather than the individual languages. Indeed, it was one of the innovations defining that branch. This fact would be missed if the comparanda of the Uralic family included as primary data the "de-graded" states of Finnish, Estonian, and Sami.  
Not all synchronic alternation is amenable to internal reconstruction. Even if cases of secondary split (see phonological change) often result in alternations that signal a historical split, the conditions involved are usually immune to recovery by internal reconstruction. For example, the alternation of voiced and voiceless fricatives in Germanic languages, as described in Verner's law, cannot be explained only by examination of the Germanic forms themselves.
That a general characteristic of secondary split, but occasionally, internal reconstruction can work. Primary split in principle is recoverable by internal reconstruction whenever it results in alternations, but later changes can render the conditioning irrecoverable.
English has two patterns for forming the past tense in roots ending in apical stops, i.e. /t d/.
Although modern English has very little affixal morphology, its number includes a marker of the preterite, apart from verbs with vowel changes of the find/found sort, and the great majority of verbs that end in /t d/ take /ɪd/ as the marker of the preterite, as seen in Type I.
Can we make any generalizations about the membership of verbs in Types I and II? Most obviously, the Type II verbs all end in /t/ and /d/, though that is just like the members of Type I. Less obviously they are all without exception basic vocabulary. Note well that this is a claim about Type II verbs and not a claim about basic vocabulary: there are basic home-and-hearth verbs in Type I, too. But there are no denominative verbs in Type II, that is, verbs like to gut, to braid, to hoard, to bed, to court, to head, to hand. There are no verbs of Latin or (a little harder to spot) of French origin; all stems like depict, enact, denote, elude, preclude, convict are Type I. Furthermore, all novel forms are inflected as Type I: all native speakers of English would presumably agree that the preterites of to sned and to absquatulate would most likely be snedded and absquatulated.
The inference from these considerations is that the absence of a "dental preterite" marker on roots ending in apical stops in Type II reflects a more original state of affairs, i.e., that in the early history of the language the "dental preterite" marker was in a sense absorbed into the root-final consonant when it was /t/ or /d/; the affix /ɪd/ after word-final apical stops then belongs to a later stratum in the evolution of the language. The same suffix is involved in both types, but with a 180° reversal of "strategy": other exercises of internal reconstruction would point to the conclusion that the aboriginal affix of the dental preterites was /Vd/ (where V = a vowel of uncertain phonetics, and of course an inspection of Old English directly would reveal several different stem-vowels in the mix). In modern formations, it is stems ending in /t d/ that preserve the vowel of the preterite marker; in an earlier day, odd as it might seem, the loss of the stem vowel had taken place already prehistorically whenever the root ended in an apical stop.
In Latin there are many examples of "word families" showing vowel alternations. Some of these are examples of Indo-European ablaut: pendō "weigh", pondus "a weight"; dōnum "gift", datum "a given", caedō "cut" perf. ce-cid-, dīcō "speak", participle dictus, that is, inherited from the proto-language. (Note: all unmarked vowels in these examples are short.) But some, involving only short vowels, clearly arose within Latin. Examples:
faciō "do", participle factus, but perficiō, perfectus "complete, accomplish"; amīcus "friend" but inimīcus "unfriendly, hostile"; legō "gather", but colligō "bind, tie together", participle collectus; emō "take; buy", but redimō "buy back", participle redemptus; locus "place" but īlicō "on the spot" (< *stloc-/*instloc-); capiō "take, seize", participle captus but percipiō "lay hold of", perceptus; arma "weapon" but inermis "unarmed"; causa "lawsuit, quarrel" but incūsō "accuse, blame"; claudō "shut", inclūdō "shut in"; caedō "fell, cut", but concīdō "cut to pieces"; damnō "find guilty" but condemnō "sentence" (verb); and many, many more of the same sort. Briefly: vowels in initial syllables never alternate in this way, but in non-initial syllables (omitting some details) short vowels of the simplex forms become -i- before a single consonant and -e- before two consonants; the diphthongs -ae- and -au- of initial syllables alternate with medial -ĩ- and -ũ-, respectively.
Now, reduction in contrast in a vowel system (for that is what has happened here) is very commonly associated with position in atonic (unaccented) syllables, but in Latin the tonic accent of say reficiō and refectus is on the same syllable as simplex faciō, factus, and that is true of almost all of the examples given (cólligō, rédimō, īlicō (initial-syllable accent) are the only exceptions), and indeed for most of the examples of these alternations throughout the language. Obviously the reduction of contrast points in the vowel system (-a- and -o- fall together with -i- before a single consonant, with -e- before two consonants; long vowels replace diphthongs) cannot have anything to do with the location of the accent in attested Latin:
The accentual system of Latin is well-known, partly from statements by Roman grammarians and partly from agreements among the Romance languages on the location of tonic accent: the tonic accent in Latin fell three syllables before the end of any word with three or more syllables, unless the second-to-last syllable (called the penult in classical linguistics) was "heavy", i.e. contained a diphthong, a long vowel, or was followed by two or more consonants, in which case that syllable has the tonic accent. Thus perfíciō, perféctus, rédimō, condémnō, inérmis.
If there is any connection, then, between word-accent and vowel-weakening, the accent in question cannot be that of Classical Latin. Since the vowels of initial syllables never show this weakening (to oversimplify a bit), the obvious inference is that at some point in prehistory, the tonic accent must have been a "stationary" accent always falling on the first syllable of a word. Such an accentual system is very common in the world's languages ( Czech, Latvian, Finnish, Hungarian, and, with certain complications, High German and Old English, etc., etc.), but was definitely not the accentual system of Proto-Indo-European. Therefore, on the basis of internal reconstruction within Latin, we discover a prehistoric sound-law that replaced the inherited accentual system with an automatic initial-syllable accent which, in turn, was replaced by the attested accentual system. As it happens, Celtic languages likewise have an automatic word-initial accent (subject, like the Germanic languages, to certain exceptions, mainly certain pretonic prefixes). Celtic, Germanic, and Italic languages share some other features as well, and it is tempting to think that the word-initial accent system was an "areal feature" as it is called, but that would be more speculative than the inference of a prehistoric word-initial accent for Latin specifically.
There is a very similar set of givens in English, but with very different consequences for internal reconstruction. There is pervasive alternation between long and short vowels (the former now phonetically diphthongs): between // and // in words like divide, division; decide, decision; between // and // in words like provoke, provocative; pose, positive; between // and // in words like pronounce, pronunciation; renounce, renunciation; profound, profundity; and many other examples. As in the Latin example, the tonic accent of present-day English is often on the syllable showing the vowel alternation. In the Latin case, it was possible to frame an explicit hypothesis regarding the location of word-accent in prehistoric Latin that would account for both the vowel alternations and the attested system of accent. Indeed, such a hypothesis is hard to avoid. By contrast, the alternations in English do not point to any specific hypothesis, only a general suspicion that word accent must be the explanation, and that the accent in question must have been different from that of modern-day English. Where the accent used to be, and what the rules (if any) are for its relocation in present-day English, cannot be recovered by internal reconstruction. In fact, even the givens are uncertain: it is not even possible to tell whether we are dealing with lengthening in tonic syllables or shortening in atonic ones. (And in historical fact, both are involved.) Part of the problem is that English has alternations between diphthongs and monophthongs (i.e., between Middle English long and short vowels, respectively) from no fewer than six different sources, the oldest (for example write, written) dating all the way back to Proto-Indo-European. But even if it were possible (it is not) to sort out the corpus of affected words, sound changes subsequent to the relocation of tonic accent have eliminated the necessary conditions for framing accurate sound laws. It is, in fact, possible to reconstruct the history of the English vowel system with great accuracy, but not by internal reconstruction. (In a nutshell, at the time of the atonic shortening, the tonic accent lay two syllables to the right of the affected vowel and was subsequently retracted to its present-day position. But in words like division and vicious (cf. vice) have lost a syllable in the first place, which would be an insuperable obstacle to correct analysis.)
- Smith, Jennifer L. (2012-10-31). "LING 202 Lecture Outline" (PDF). The University of North Carolina at Chapel Hill (PDF). p. 5. Retrieved 7 January 2014.
- Campbell, Lyle (2013). Historical Linguistics (3rd ed.). Edinburgh University Press. p. 199. ISBN 978-0-7486-7559-3.
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- Campbell (2013), pp. 211–212.
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