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Is Proto-Balto-Slavic zero-grade from long zero-grade i
pílˀnas
wilkás
źírˀna
śírˀnāˀ

Is Proto-Germanic zero-grade from long zero-grade u
fullaz
wulfaz
kurną
hurną

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As three seconds of looking it up (probably in the same place you found those forms to begin with) shows:

  • PBS *pílˀnas < PIE *pl̥h₁nós > PGmc *fullaz
  • PBS *wilkás < PIE *wĺ̥kʷos > PGmc *wulfaz
  • PBS *źírˀna < PIE *ǵr̥h₂nóm > PGmc *kurną
  • PBS *śírˀnāˀ < PIE *ḱr̥h₂-néh₂, *ḱr̥h₂-nó-m > PGmc *hurną

So evidently the *i and *u are epenthetic and the result of the devocalisation of the vocalic resonants. That is to say, Proto-Indo-European *l̥ and *r̥ represent /l/ and /r/ functioning as vowels, which is something neither Proto-Germanic nor Proto-Balto-Slavic likes, so they convert them to consonants and add a vowel to avoid the loss of the syllable and further pronunciation difficulties.

"Zero-grade i" and "zero-grade u" aren't meaningful terminology. Your table shows the behaviour of an ablauting vowel and a following semivowel in Proto-Balto-Slavic; when the ablauting vowel is present, those semivowels exist as glides, and when it's not (i.e. when it's in the zero-grade), they turn into vowels. In this case the roots of these words are in the zero-grade, but they don't have following semivowels, so it's not relevant.

(There are other languages that deal with the *l̥ in *wĺ̥kʷos by vocalising the *w instead: Proto-Celtic *ulkos and Albanian ulk, for instance.)

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  • PIE l̥, r̥ ~∅l, ∅r zero it's obvious without any examples – fedor Apr 9 at 6:19
  • @fedor I'm not sure what you're trying to say. It's extremely common for PIE vocalic resonants to become consonants with epenthetic vowels. It happens at least some of the time in most daughter languages. – Cairnarvon Apr 9 at 11:24

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