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 had 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" () singular accusatives, of r-stems, < "sister" acc. singular of r-stems, < "hand" acc. singular of s-stems, < "Ausos" (Vedic Sanskrit )
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 (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.
See also Glossary of sound laws in the Indo-European languages
References G. Klingenschmitt (1992), p. 44. M. Kümmel, (1996), p. 9. K.-H. Mottausch, HS 113 (2000). K.-H. Mottausch, HS 114 (2001). H. Rix, sūdor and sīdus in: FS Knobloch (ed. Ölberg, 1985), p. 348 K. Stüber, (2002), p. 24f.