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The *kʷetwóres rule of Proto-Indo-European (PIE) is a sound law of PIE accent, stating that in a word of three syllables é-o-X the accent will be moved to the penultimate, e-ó-X. It has been observed by earlier scholars, but it was only in the 1980s that it attracted enough attention to be named, probably first by Helmut Rix in 1985. Examples:

• *kʷetwóres < *kʷétwores "four" (Latin: quattuor)
• singular accusatives,
  • of r-stems, *swes-ór-m̥ < *swés-or-m̥ "sister" acc. singular
  • of r-stems, *ǵʰes-ór-m̥ < *ǵʰés-or-m̥ "hand" acc. singular
  • of s-stems, *h₂ews-ós-m̥ < *h₂éws-os-m̥ "Ausos" (Vedic Sanskrit uṣā́sam)

The rule is fed by an assumed earlier sound law that changes è to ò after an accented syllable: *kʷetwóres < *kʷétwores < *kʷétweres.

Rix invoked the rule in the 1998 preface to the Lexikon der indogermanischen Verben (p. 22) to explain why in the PIE perfect the root ó grade is accented: *ǵe-ǵónh- / *ǵé-ǵn̥h- < *ǵé-ǵenh- / *ǵé-ǵn̥h- "created/engendered".

The rule has been invoked by Mottausch to explain accented ó grades in the PIE nominal ablaut.

• G. Klingenschmitt Die Lateinische Nominalflexion (1992), p. 44.
• M. Kümmel, Stativ und Passivaorist (1996), p. 9.
• K.-H. Mottausch, Die idg. Nominalflexion und die o-Stufe HS 113 (2000).
• K.-H. Mottausch, Die thematischen Nomina im Idg. HS 114 (2001).
• H. Rix, sūdor and sīdus in: FS Knobloch (ed. Ölberg, 1985), p. 348
• K. Stüber, Die primären s-Stämme (2002), p. 24f.

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