Holtzmann's Law

Holtzmann's law is a Proto-Germanic sound law originally noted by Adolf Holtzmann in 1838. It is also known by its traditional German name Verschärfung (literally: "sharpening"). (A similar sound law which has affected modern Faroese, called skerping in Faroese itself, is also known as "Faroese Verschärfung" in English.)

The law involves the gemination, or doubling, of PIE semivowels (glides) * -y- and * -w- in strong prosodic positions into Proto-Germanic * -jj- and * -ww-, which had two outcomes:

The process is brought about by the fact that vowels (or semivowels) in the syllable margin are invariably transformed into consonantal articulations.

The conditions of the sound change were long debated, since there was a seemingly random distribution of affected and unaffected words. At first, dependence on word accent was assumed, parallel to Verner's Law. One currently accepted solution, first proposed by Smith (1941), postulates dependency on the presence of a PIE laryngeal, which when lost, triggered lengthening as if the semivowels were vowels, and forced them into the syllable margin.

According to Lehmann (1955), the lengthening occurs in the contexts of PIE * -VwH-, * -iyH-, * -ayH-, * -aHy- (where V is any short vowel, and H is any laryngeal).

...
Wikipedia