Siebs' law is a Proto-Indo-European

Proto-Indo-European language

The Proto-Indo-European language is the reconstructed common ancestor of the Indo-European languages, spoken by the Proto-Indo-Europeans...

 (PIE) phonological

Proto-Indo-European phonology

The phonology of the Proto-Indo-European language has been reconstructed by linguists, based on the similarities and differences among current and extinct Indo-European languages...

 rule named after the German linguist Theodor Siebs

Theodor Siebs

Theodor Siebs was a German linguist most remembered today as the author of Deutsche Bühnenaussprache published in 1898...

. According to this law, if an s-mobile is added to a root that starts with a voiced or aspirated stop, that stop is devoiced.
Compare:

PIE > Latin

Latin

Latin is an Italic language originally spoken in Latium and Ancient Rome. It, along with most European languages, is a descendant of the ancient Proto-Indo-European language. Although it is considered a dead language, a number of scholars and members of the Christian clergy speak it fluently, and...

 fragor,

but > PIE > Sanskrit

Sanskrit

Sanskrit , is a historical Indo-Aryan language and the primary liturgical language of Hinduism, Jainism and Buddhism.Buddhism: besides Pali, see Buddhist Hybrid Sanskrit Today, it is listed as one of the 22 scheduled languages of India and is an official language of the state of Uttarakhand...

 sphūrjati.

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 Szemerenyi

Oswald Szemerényi

Oswald John Louis Szemerényi was a Hungarian Indo-Europeanist with strong interests in comparative linguistics in general....

 has rejected this rule, explaining that it is untenable and cites the contradiction present in Avestan zdī from PIE "be!" as counterproof (Szemerenyi 1999: 144). However, the PIE form is more accurately reconstructed as and thus Siebs' law appears to demand that the sibilant and aspirated stop are both adjacent and tautosyllabic

Tautosyllabic

Two or more phonemes are tautosyllabic if they occur in the same syllable. Take for instance the English word "cat". Since this word is monosyllabic, the three phonemes /k/, /æ/ and /t/ are tautosyllabic. They can also be described as sharing a 'tautosyllabic distribution'...

, something which is known to only occur in word-initial position in Proto-Indo-European anyway.