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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:

PIE *bʰr̥Hg- > Latin fragor,

but *s-bʰr̥Hg- > PIE *spʰr̥Hg- > Sanskrit sphūrjati.

Siebs proposed this law in the Zeitschrift für vergleichende Sprachforschung auf dem Gebiete der indogermanischen Sprachen, as Anlautstudien.^[1] Oswald Szemerényi rejected this rule, explaining that it is untenable and cites the contradiction present in Avestan zdī from PIE *s-dʰi "be!" as counterproof.^[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.

• Collinge, N. E. (1985). The Laws of Indo-European. John Benjamins Publishing Co. ISBN 978-0-915027-75-0.
• Szemerényi, Oswald (1999). Introduction to Indo-European Linguistics. Oxford University Press. ISBN 978-0-19-823870-6.
• Siebs, Theodore (1904). Zeitschrift für vergleichende Sprachforschung auf dem Gebiete der indogermanischen Sprachen.