*kʷetwóres rule

The ' 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:


 * ' < ' "four" (quattuor)
 * singular accusatives,
 * of r-stems, ' < ' "sister" acc. singular
 * of r-stems, ' < ' "hand" acc. singular
 * of s-stems, ' < ' "Ausos" (Vedic Sanskrit uṣā́sam)

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

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: <  "created/engendered".

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