Ablaut

Everyone’s favourite apophony, already lying on the boundary between phonology and morphology in PIE.

Overview

o-grade rule
- Underlying e or ∅ are changed to the o-grade in a disjunct set of morphological environments (==PIE strong cases, sg. active indicative==).
- It would also seem that o replaces ∅ where the latter would be otherwise inadmissible, e.g. acc. sg. swésor-ṃ ‘sister’, though the reason is not clear here.
- Similarly pretonic root syllables for derived causative verbs take the o-grade, e.g. 3sg. woséyeti ‘clothes sb‘.¹
∅-grade rule Surface unaccented ablauting vowels would often be deleted, especially in stems but also in derivations. Exceptions are numerous:
- Unaccented ablauting vowels are visible in pédes ‘of a foot’ (cf. Latin pedis) and nébʰesos ‘of a cloud’ (cf. Homeric Greek νέφεος);
- Accented ∅ can likewise be found in wḷ́kʷos ‘wolf’ and h₂ṛ́tkos ‘bear’. So the rule is already partially morphologised in PIE. ¹
lengthened grade rule Some nominals exhibit a lengthened grade in the strong cases, and verbs in the indicative singular active. See also PIE vowel lengthening by contraction.

The PIE thematic vowel followed a special version of the o-grade and ∅-grade rules, listed in that note.

In part due to PIE *e-laryngeal colouring and the loss of laryngeals in most daughter languages, the ablaut system had to be reinterpreted in all daughter languages.