Cowgill's law says that a PIE laryngeal /h₃/, and possibly /h₂/, turns into /k/ in Proto-Germanic when directly preceded by a sonorant and followed by /w/. This law is named after Indo-Europeanist Warren Cowgill.
This law is still controversial, although increasingly accepted. Donald Ringe (2006) accepts it; Andrew Sihler (1995) is noncommittal.
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As the Proto-Indo-European language (PIE) broke up, its sound system diverged as well, as evidenced in various sound laws associated with the daughter Indo-European languages.Especially notable is the palatalization that produced the satem languages, along with the associated ruki sound law. Other notable changes include:
Bartholomae's law in Indo-Iranian, and Sievers's law in Proto-Germanic and (to some extent) various other branches, may or may not have been common Indo-European features. A number of innovations, both phonological and morphological, represent areal features common to the Italic and Celtic languages; among them the development of labiovelars to labial consonants in some Italic and Celtic branches, producing "p-Celtic" and "q-Celtic" languages (likewise "p-Italic" and "q-Italic", although these terms are less used). Another grouping with many shared areal innovations comprises Greek, Indo-Iranian, and Armenian; among its common phonological innovations are Grassmann's law in Greek and Indo-Iranian, and weakening of pre-vocalic /s/ to /h/ in Greek, Iranian and Armenian.