Kluge's law is a controversial Proto-Germanic sound law formulated by Friedrich Kluge. It purports to explain the origin of the Proto-Germanic long consonants*kk, *tt, *pp and *rr (Proto-Indo-European lacked a length distinction for consonants) as originating in the assimilation of an n to a preceding voiced consonant, under the condition that the n was part of a suffix which was accented in the ancestral Proto-Indo-European (PIE). The name "Kluge's law" was coined by Frederik Kortlandt. This law is not generally accepted by historical linguists.
Note that in some examples, like *bulkaz and *deupaz, the geminate simplifies, presumably because it followed a heavy syllable. However, this did not apply in every such case, as seen in the following example:
PGmc *hwīt-, *hwitta- < PIE *ḱwéit-, *ḱwitnó- (Skt. śvítna-) "white".
English and German regularized the variant with long vowel and without geminate (white; Weiss), though with the voiceless *t, a result of Kluge's law, whereas Dutch regularized the variant with short vowel and geminate (wit, witte).
This section requires expansion with: modern reflexes. (August 2013)
Kluge's law had a noticeable impact on Proto-Germanic morphology, as it gave rise to alternation of geminated and non-geminated consonants, in both nominal and verbal paradigms. These alternations are typologically similar to the well-known paradigmatic interchanges of consonant strength in the neighboring Finnish and other Finno-Ugric languages, known as consonant gradation. Guus Kroonen (2011) extends this name to the Proto-Germanic consonant alternations resulting from Kluge's Law as well.
The law has sparked[when?] discussions about its chronology in relation to Grimm's law and Verner's law. The problem is that the traditional ordering (1. Grimm, 2. Verner, 3. Kluge) can not account for the absence of voice in the Proto-Germanic geminates. It has therefore been proposed[according to whom?] to rearrange the order of events so that the Proto Germanic geminates' loss of voice may be equated with that part of Grimm's law that turns mediae into voiceless tenues. This would mean that Kluge's law happened before (or between different phases of) Grimm's law. If accepted, this has further consequences, because Verner's law must in fact precede Kluge's law, or otherwise it can not be explained why both the reflexes of PIE voiced aspirated plosives and PIE voiceless plosives underwent Kluge's law. Consequently, this would put Verner's law chronologically in the first position, followed by Kluge's and finally Grimm's law.
Under the updated view, the processes may be summarized by the following table:
All three sets of stops occur before accented suffixes.
Voiceless stops occurring after an unaccented syllable are voiced.
Stop + *n becomes, before an accented vowel, a geminate.
Grimm's law and stress shift
Voiced stops are devoiced, and accent is shifted on the initial syllable.