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.
[1] The conditions of the sound change were long debated, since there was a seemingly random distribution of affected and unaffected words.
One currently accepted solution, first proposed by Smith (1941),[2] 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.
For example, PIE *drewh₂yo → early Proto-Germanic *trewwjaz 'trustworthy, faithful' →: One instance where a laryngeal was never present is PIE *h₂ōwyóm 'egg', but after the loss of * -w-, the * -y- shifted into the syllable margin, giving: Some linguists (e.g. Joseph Voyles)[5] hold 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 W. Marchand's[6] observation that a Runic inscription (niuwila on the Naesbjaerg bracteate of the 5th century) and an early loan into Finnic (*kuva 'picture', cf.