Laryngeal theory in the context of "Cowgill's law (Germanic)"

The laryngeal theory is a widely accepted scientific theory in historical linguistics positing that the Proto-Indo-European (PIE) language included a series of consonants that left no direct consonantal descendants in languages outside of the Anatolian branch. It was first proposed by the Swiss linguist Ferdinand de Saussure in 1878 to explain apparent irregularities in morphophonological patterns in daughter languages. At the time no direct evidence for the existence of such sounds was available; however, the theory allowed for a better reconstruction of PIE ablaut and root. This changed in 1927 when a Polish linguist Jerzy Kuryłowicz discovered that a sound transcribed as ḫ in the newly deciphered ancient Indo-European Hittite language appears in many of the places that the laryngeal theory predicted.

Subsequent scholarly work has established a set of rules by which an ever-increasing number of reflexes in daughter languages may be derived from PIE roots. The number of explanations thus achieved and the simplicity of the postulated system have both led to widespread acceptance of the theory.