The comparative method is used by historical linguists to reconstruct unattested languages such as Proto-Indo-European and Proto-Germanic. Historical linguists employing the comparative method arrive at their conclusions by comparing languages that have evolved from the unattested language, in the case of English, Proto-Germanic, and taking note of their differences and simliarities. By analyzing these differences and similiarities, it is possible to form a reasonable notion of the morphology, syntax and phonology of an extinct language that has left no physical traces. For instance, by comparing the words for the number 2 in a number of descendant languages and comparing their similarities and differences, it is possible to reconstruct what the word for the number 2 might have been in the language that serves as the common ancestor for the languages being compared. For instance, in English, this is represented by the word two, in Latin, duo, in German, zwei, by comparing these modern forms it is possible for historical linguists to deduce, with reasonable certainty, that the Indo-European word for the number 2 might have been dwo.

Grimm’s Law describes the changes that resulted in the important differences between Germanic and other Indo-European languages. Grimm’s Law states that voiceless stops [p,t,k] became fricatives [f,θ,x]; voiced stops [b,d,g] became voiceless stops [p,t,k]; and voiced aspirated stops [bʰ,dʰ,gʰ] became non-aspirated voiced stops [b,d,g] through intermediate steps. Grimm’s Law describes changes that took place sometime between 3000 B.C.E. and 100 B.C.E., each of these changes taking place over several hundred years and following in sequence. Each of the three stages posited by Grimm’s Law occurred in sequence and took hundreds of years. For example, in the first stage, voiceless stops [p,t,k] become voiceless fricatives [f,θ,x]. Thus, the Indo-European *pəter ‘father’ became the Germanic *fadar- ‘father’. Similarly, the Indo-European *dent- ‘tooth’ became the Germanic tanth ‘tooth’. The first stage, in Germanic languages, was followed by the changes described by Verner’s Law. In the second stage of Grimm’s Law, voiced stops [b,d,g], were devoiced to [p,t,k]. Thus, the Indo-European leb- ‘lip’ became the Germanic lep- ‘lip’ and the Indo-European ped- ‘foot’ devoiced to fot- ‘foot’. Note in the previous example that the p of the Indo-European word had already been changed to the f in the Germanic by the first stage described by Grimm’s Law. The final stage of Grimm’s Law involves the deaspiration of aspirated voiced stops [bʰ,dʰ,gʰ] to the unaspirated equivalents, [b,d,g]. For example, the Indo-European ghabh- ‘give or receive’ becomes Germanic giban ‘to give’, with two instances of deaspiration occurring. Similarly, Indo-European dhwer- ‘door’ becomes Germanic *durunz.

The first stage of Grimm’s Law, voiceless stops [p,t,k] becoming voiceless fricatives [f,θ,x], is not consistent in all instances in Germanic languages. Instead, specific phonological enviornments triggered the voiceless stops [p,t,k] to become voiced fricatives [ß,ð,ɣ] respectively. Three phonological environments block the observations stated by Verner’s Law: if the sounds occur word-initially, if the sound occurs next to a voiceless sound or the Indo-European stress falls on the syllable immediately preceeding the sound. Thus, if any of these enviornments are present, Verner’s Law is blocked. For example, the Indo-European word pəter which developed into the Latin pater should have, according to Grimm’s Law, appeared in Germanic as [fɑθer]; however, since the word in Indo-European is stressed on the second syllable and [θ] occurs word-medially and intervocallically, [θ] voices to [ð] resulting in the Modern English father. Therefore, Verner’s Law could be stated that Proto-Germanic voiceless fricatives became voiced when they were in a voiced environment and the Indo-European stress was not on the immediately preceding syllable. Because later changes in Proto-Germanic caused the stress of the Indo-European word to move to the first syllable, the effects of Verner’s Law were obscured and viewed as anomalies to Grimm’s Law.