Siebs's law (/ˈziːps/ ZEEPS) is a Proto-Indo-European (PIE) phonological rule named after the German linguist Theodor Siebs.
Compare: Siebs proposed this law in the Zeitschrift für vergleichende Sprachforschung auf dem Gebiete der indogermanischen Sprachen ('Journal for Comparative Linguistic Research in the Field of Indo-European Languages'), as Anlautstudien ('Initial Sound Studies').
[1] Oswald Szemerényi rejected this rule, explaining that it is untenable and cites the contradiction present in Avestan zdī from PIE *s-dʰi "be!"
[2] However, the PIE form is more accurately reconstructed as *h₁s-dʰí from *h₁es- (so not an s-mobile) and thus Siebs' law appears to demand that the sibilant and aspirated stop are both adjacent and tautosyllabic, something which is known to only occur in word-initial position in Proto-Indo-European anyway.
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