o-grade: Verbal nouns stem-stressed masculine action nouns (Greek gónos "offspring", Sanskrit jánas "creature, person"; Greek trókhos "circular course" < "*act of running"); ending-stressed feminine, originally collective, action nouns (Greek gonḗ "offspring", Sanskrit janā́ "birth"); ending-stressed masculine agent nouns (Greek trokhós "wheel" < "*runner"). Nominative, vocative and accusative singular of certain nouns (acrostatic root nouns such as dṓm, plural dómes "house"; proterokinetic neuter nouns such as *wódr̥ "water" or dóru "tree"). Present tense of causative verbs; stem (not root) stress. Perfect singular tense.
o-grade: Verbal nouns stem-stressed masculine action nouns (Greek gónos "offspring" Sanskrit jánas "creature person"; Greek trókhos "circular course" < "*act of running"); ending-stressed feminine, originally collective, action nouns (Greek gonḗ "offspring", Sanskrit janā́ "birth"); ending-stressed masculine agent nouns (Greek trokhós "wheel" < "*runner"). Nominative, vocative and accusative singular of certain nouns (acrostatic root nouns such as dṓm, plural dómes "house"; proterokinetic neuter nouns such as *wódr̥ "water" or dóru "tree"). Present tense of causative verbs; stem (not root) stress. Perfect singular tense.