Achievement of an understanding of the vowel system and its variations known as ablaut provided the key to reconstructing earlier stages of Proto-Indo-European. Through that achievement, probably the greatest difficulty in Indo-European studies was overcome. In the process the results of earlier sound changes had to be examined for their interrelation ships. These were obscured by subsequent changes, such as umlaut. They were also difficult to untangle because the short vowel system of Proto-Indo-European, as late as Schleicher’s Compendium, was assumed to consist of the three vowels a i u in keeping with those of Sanskrit. In comparing these with the vowels of the other dialects a was found to correspond to the five vowels / e a o w, but no pattern of correspondence could be determined. Moreover, the ablaut interchanges had been utilized to distinguish mor
phological categories, and accordingly extended to new patterns; for that reason the phonological environments in which the vowel changes giving rise to ablaut had taken place were determined only slowly and through painstaking analysis. Additional problems were caused by the losses of laryngeals, which were members of the phonological system when the original ablaut changes had taken place. Solving these problems to achieve an understanding of ablaut required the efforts of several generations of Indo-Europeanists. The course of their work illustrates the gradual improvement of linguis
tic methodology, including the importance of appropriate formalism. Before we reconstruct the earlier phonological system some of the import ant findings in that effort will be briefly sketched here as examples of successive refinements of the theoretical bases by which our current views of the proto-language and its earlier stages were achieved.