IntroductionInternal Reconstruction is an established method in historical linguistic research. Joseph (2010) gives a pretty concise overview of the merits and limits of this method. We can trace it back to the comparative method of historical linguistics in the first part of the nineteenth century (see Baxter 2002, and see Chapter 9, this volume). It concentrates mainly on phonological and morphological changes (or morphophonemic changes) on the basis primarily of written language data. The reasoning is in principle by analogy: in a given language L, if a form A has a grammatical relation to a form A′ and we find a lexical entity B of the same lexical category as A but no corresponding B′, we conclude that there might have been B’ in an earlier stage of the language L. As an illustration, we report Joseph's (2010: 53f.) example of Ancient (Attic) Greek. The masculine noun in the nominative singular poimé:n ‘shepherd’ has the genitive singular form poiménos, the feminine noun in the nominative singular óar ‘wife’ has the genitive singular form óaros. Now, in Ancient Greek we find the genitive singular form mélitos ‘of the honey’ but in the nominative singular we do not find *mélit but just méli ‘honey’ (neuter). From this Joseph concludes that in an earlier stage of Ancient (Attic) Greek (or we might even say in its Proto-Indo-European ancestor) most probably there existed *mélit in the nominative singular and that by regular sound change the final stop underwent apocope. There are several other nouns in Ancient (Attic) Greek which have the same stem allomorphy in their inflectional paradigm: sô:ma (neuternom.sg) ‘body’ ~ sómatos (gen.sg). Indeed, in Ancient (Attic) Greek there are no words ending in a voiceless dental (or alveolar) stop. It seems obvious that this language-specific phonological well-formedness constraint on word formation brought about a regular rule, namely the apocope of word final stops. Yet, this regular sound change results in stem allomorphy in the inflectional paradigm of some nouns. This illustrates what is meant by morphophonemic change, or to put it more succinctly, regular phonological change provokes irregularity in inflectional morphology. But how do we know that there must have been a stage in Ancient Greek where there existed words with final stops if we cannot find any evidence in the data?