Originally, the noun was a "regular" i-stem, and would have been *rēj- before vowels (genitive *rējes, dative *rējei etc.), and *rēi before consonants and word-finally (nominative *rēis, perhaps originally disyllabic). In the former, -j- was regularly lost, while in the latter the diphthong was shortened before another consonant, due to Osthoff's law.
|vowel/i-stem, irregularDeclension of *reis (vowel/i-stem, irregular)|
|accusative||*reim, *rēm||*reins? *rens?|
|ablative||*rīd?, *rē?||*reiβos (*rēβos?)|
- Latin: rēs