Abstract
Gradation (= apophony) or ablaut is a morphologically induced vowel change between the zero-grade (= no ablaut vowel), full grade (ablaut vowels e or o), and lengthened grade (ablaut vowels ē or ō). Due to phonological conservatism, the PIE gradation e/o/zero was well preserved in Greek roots while the lengthened grade was mostly given up. In derivational suffixes ē and ō ablaut was retained and even spread.