The so-called rule of Proto-Indo-European (PIE) has been observed by earlier scholars, but has only recently attracted enough attention to be named, probably first by Helmut Rix in 1985. It 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. Examples include

• < "four"
• singular accusatives,
• of r-stems, < "sister" Acc. Sg.
• of r/n-heteroclitica, < "hand" Acc. Sg.
• of s-stems, < "Hausos" (Vedic Sanskrit uṣā́sam'')

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

Rix invokes 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, e.g. < "created/engendered".

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

References

• 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.