||The Proto-Indo-European long vowels *ē and *ō occupy a remarkable position within the phonemic system. Although these vowels are phonemic, they are limited to very specific morphological categories. This distribution has been explained by several theories, of which there are three which propose a phonetic origin for these long vowels and that nowadays find supported by various scholars, viz. Wackernagel’s lengthening in monosyllables, Szemerényi’s Law, and Kortlandt’s lengthening before word-final resonant. These three theories have in common that they derive the long vowels from their short counterparts *e and *o, whereas they differ from each other in the phonological environments under which the short vowels would have become long. It is, however, still controversial which theory is the most likely to be correct, since all theories have counterexamples. This thesis examines the question which of the three phonetic theories on the origin of the Proto-Indo-European lengthened grade can be proven correct or incorrect.
This question will be addressed by discussing the evidence and counterevidence of the nominal system and comparing the counterexamples to the three theories. By attempting to provide alternative explanations for the counterevidence, as well as discussing the strengths and weaknesses of existing alternative explanations, it is possible to examine which theory or theories can be kept up and which one(s) must be rejected.
It will be concluded, that monosyllabic lengthening probably works for the nominal system, that Kortlandt’s lengthening before word-final resonant can only work when it is reformulated(i.e. leaving out the nasals as a conditional factor), and that Szemerényi’s Law is best to be given up.