Siebs's law

Siebs's law is a Proto-Indo-European phonological rule named after the German linguist Theodor Siebs. According to this law, if an s-mobile is added to a root that starts with a voiced or aspirated stop, that stop is allophonically devoiced.
Compare:

Discussion

Siebs proposed this law in the Zeitschrift für vergleichende Sprachforschung auf dem Gebiete der indogermanischen Sprachen, as Anlautstudien. Oswald Szemerényi rejected this rule, explaining that it is untenable and cites the contradiction present in Avestan from PIE *s-dʰi "be!" as counterproof. However, the PIE form is more accurately reconstructed as *h₁s-dʰí from *h₁es- 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.