ABSTRACT
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 interrelationships. 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 i e a o u, but no pattern of correspondence could be determined.
