Proponents of Kluge's law use this to explain why so many Proto-Germanic roots (especially of strong verbs) end in short *p, *t, or *k even though their likely cognates in other Indo-European languages point to final Proto-Indo-European (PIE) consonants other than the expected *b, *d, *g, or *ǵ.
Because of its dependence on ablaut and accent, it operated in some parts of declension and conjugation, but not in others, giving rise to alternations of short and long consonants in both nominal and verbal paradigms.
This is most conspicuous in the n-stem nouns and the "néh₂-presents" (imperfective verbs formed from perfective ones by adding the PIE suffix *néh₂-/*nh₂-), but also occurs in mn-stems and directional adverbs.
With rather few exceptions, introductory texts have ignored it, and more detailed works on Proto-Germanic have generally dismissed it rather briefly; "it has been seriously challenged throughout the 20th century, and nowadays even borders on the uncanonical in both Indo-European and Germanic linguistics" (Kroonen 2009: 53).
Long consonants more generally are ubiquitous in Germanic nicknames such as Old English Totta from Torhthelm, Beoffa from Beornfriþ, Blæcca for a black-haired man (note the short /k/ in blæc), Eadda (and German Otto) from all names with Proto-Germanic *Auda- (Gąsiorowski 2006[14] and references therein), a long list of Gothic ones whose referents are often difficult or impossible to reconstruct (Ibba, Faffo, Mammo, Oppa, Riggo, Wacca, etc.
; possibly also atta, meaning "father"), German ones such as – accounting for the High German consonant shift – Fritz (*Fritta(n)-) from Friedrich, Lutz (*(H)lutta(n)-) from Ludwig, and Sicko (*Sikkan-) from Si(e)gmar, and finally Icelandic Solla from Sólrún, Magga from Margrét, Nonni from Jón, Stebbi from Stefán, Mogga from Morgunblaðið, and lögga "cop" from lögreglan "the police";[6] Gąsiorowski (2006)[14] further proposed to explain the otherwise enigmatic English words dog, pig, frog, stag, (ear)wig, and Old English sucga "dunnock" and *tacga ~ *tecga "young sheep" (not attested in the nominative singular) as nicknames formed to various nouns or adjectives.
Some authors, such as Trautmann (1906)[10] and Fagan (1989),[12] have tried to ascribe all long plosives of Proto-Germanic to "intensive" or "expressive gemination" on the basis of the idea that the roots that contained them had meanings connected to emotions, including intensity and iteration; this idea, first formulated by Gerland (1869[15] – long before Kluge published), was accepted e.g. in the extremely influential, Indogermanisches etymologisches Wörterbuch (Pokorny 1959)[16] as well as the more specialist works of Seebold (1970)[17] and Kluge & Seebold (2002),[18] and was considered "still perhaps the most widely accepted explanation" by Ringe (2006: 115).
[4] Lühr (1988)[13] and Rasmussen (1989b), approvingly cited by Kroonen (2011),[6] as well as Kortlandt (1991),[3] countered that most nouns with long plosives or evidence of consonant gradation did not have meanings that would fit this hypothesis.
In this respect, some versions of the Expressivity Theory are truly comparable to what in biology is known as Aristotle's generatio spontanea hypothesis [...], which revolved around the idea that living organisms, such as flies and eels, come about spontaneously in decaying corpses."
The hypothesis of expressive gemination has trouble explaining this, as Trautmann (1906: 66)[10] admitted while rejecting Kluge's law: "Wie wir uns freilich das Nebeneinander von z.
Similarly, Gąsiorowski (2012: 21)[19] felt that it was "methodologically unsound to invoke" "psycholinguistic factors" and other hypotheses of irregular development "until we have tried everything else", in this case, a regular sound law such as Kluge's.
Kroonen (2009: 53)[5] pointed out that, by virtue of having first been published in 1869,[15] the hypothesis of expressive gemination "basically stems from the time before the rise of the Neogrammarian doctrine of Ausnahmslosigkeit der Lautgesetze" ('exceptionlessness of the sound laws').
The reason for this change of direction was that the most important formal criterion that had been used in order to isolate non-Indo-European words from the rest of the lexicon – the Proto-Germanic geminates – turned out to be significantly overrepresented in this morphological category.
The advocates of the Leiden Substrate Theory had defined the typical Germanic cross-dialectal interchange of singulate and geminate roots as the prime indicator of prehistoric language contact.
Long plosives are very rare in the known Gothic material; other than the abovementioned nicknames (including atta), they are attested only in skatts ("money"), smakka ("fig"; n-stem) and the Latin loanword, sakkus ("sack") (Kroonen 2011).