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:
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 German: [[Lexikon der indogermanischen Verben]]|italic=yes (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.