ablaut äp´lout [key] [Ger.,=off-sound], in inflection, vowel variation (as in English sing, sang, sung, song) caused by former differences in syllabic accent. In a prehistoric period the corresponding inflected forms of the language (known through internal reconstruction) had differences in accent rather than in vowel. Phonological change resulted in alteration of syllable structure and in vowel gradation. See umlaut.
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