According to Paul Kiparsky, [2] Lachmann's law is an example of a sound law that affects deep phonological structure, not the surface result of phonological rules.
In Proto-Indo-European, a voiced stop was already pronounced as voiceless before voiceless stops, as the assimilation by voicedness must have been operational in PIE (*h₂eǵtos → *h₂eḱtos 'forced, made').
Lachmann's law, however, did not act upon the result of the assimilation, but on the deep structure *h₂eǵtos > *agtos > āctus.
Jay Jasanoff defends the Neogrammarian analysis of Lachmann's law as analogy followed by sound change.
Although this formulation ultimately derives from Ferdinand de Saussure, Jasanoff's formulation also explains problems such as: Because Lachmann's law also does not operate before PIE voiced aspirate stops, glottalic theory reinterprets the law as reflecting lengthening before glottalized stops, not voiced stops.