Siebs's law
Siebs's law is a Proto-Indo-European (PIE) 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" (Berlin, 1904, 37: 277–324). Oswald Szemerényi has rejected this rule, explaining that it is untenable and cites the contradiction present in Avestan "zdī" from PIE "be!" as counterproof (Szemerényi 1999: 144). However, the  PIE form is more accurately reconstructed as from (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.