The law involves the gemination, or doubling, of PIE semivowels ' and ' in strong prosodic positions into Proto-Germanic -jj- and -ww-, which had two outcomes:
vocalization of the first semivowel, its addition to a diphthong, and division of the diphthong and remaining semivowel into two separate segments in West Germanic.
The process is brought about by the fact that vowels 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, 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, the lengthening occurs in the contexts of PIE ', ', ', '. For example, PIE *' → early Proto-Germanic *trewwjaz 'trustworthy, faithful' →:
Some linguistshold that Holtzmann's Law represents two separate and independent sound changes, one applying to Gothic and another to Old Norse, rather than being a common innovation. This is supported by James Marchand's observation that a Runic inscription and an early loan into Finnic do not exhibit this change. If true, this would prevent Holtzmann's law being used as an example of early Gotho-Nordic unity, in which context it is often cited. Voyles's explanations of the changes do not involve laryngeal theory.
Faroese shows a similar development, where some Old Norse long vowels developed into diphthongs, which then hardened into stops, e.g. Old Norse þrír → Faroese tríggir, ON róa → Far. rógva. This phenomenon is commonly called "Faroese Verschärfung" or by the Faroese term skerping, which, however, also is used about the fronting of vowels that subsequently takes place in these contexts. Another similar change occurs in a number of Jutlandic dialects of Danish, where high vowels carrying the stød prosody develop diphthongal glides which are then "hardened" into stops or fricatives, a phenomenon commonly called "klusilspring" or "klusilparasit".
