Based on formant measurements of more than 10000 vowels from 16 German speakers, vowel quality differences between the speakers have been analyzed. The main result of this investigation is that different speakers show not only different formant values (as one would expect due to individual differences), but exhibit different arrangements in their vowel systems. These differences are demonstrated by examples from the general distribution of vowels, the structure of vowel prototypes, and the differing number of degrees of tongue height for different speakers. The problem of vowel normalization for that data will also be demonstrated and discussed.