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From Proto-Finnic *-uc. The declension has been massively reworked by analogy:

  • The oblique plural forms originally declined like those of the käsi type, but were replaced by plurals in -ks- from the vastaus-type declension, formed on the analogy of the suffix -s.
    • Words that had acquired a nominal sense early on have crossed over entirely to the vastaus declension: e.g. sisarus (sibling), originally a semantic extension from *sisarus > sisaruus (siblinghood); vanhus (old person), originally a semantic extension from *vanhus > vanhuus (oldness).
  • The long vowel appearing after consonants originates from a contraction *-ude- > *-ue- > -uu- in the genitive singular and nominative plural of bisyllabic stems, in which the third-syllable unstressed e was assimilated. This was generalized to other case forms of such words as well.
  • In trisyllabic stems such as korkeakorkeus, rakasrakkaus, the suffix would in all inflected forms gain secondary stress, preventing the loss of *-d-. From here *-d- was then reintroduced in the gen. sg. and nom. pl. of bisyllabic stems.
  • Root-final consonants in trisyllabic stems were by contrast often lost (e.g. *korkeduden > korkeuden, *rakkahuden > rakkauden), leading to the regular retention of short -u- especially after vowels.
    • The rare exceptions to this such as sisarsisaruus have however also acquired the long-vowel allomorph.


From the Proto-Indo-European perfect active participle *-wós.