Ingvaeonic nasal spirant law

In historical linguistics, the Ingvaeonic nasal spirant law (also called the Anglo-Frisian or North Sea Germanic nasal spirant law) is a description of a phonological development in some dialects of West Germanic, which is attested in Old English, Old Frisian, and Old Saxon. By this sound change, in the combination vowel + nasal + fricative, the nasal disappeared, with compensatory lengthening of the vowel. ("Spirant" is an older term for "fricative".) The sequences in question are original -ns-, -mf-, and -nþ-.

Examples

Compare the first person plural pronoun us in various old Germanic languages:

Gothic represents East Germanic, and its correspondence to German and Dutch shows it retains the more conservative form. The /n/ has disappeared in English, Frisian and Old Saxon, with compensatory lengthening of the /u/.

Likewise:

Germanic *tanþ- becomes English tooth, Old Frisian tōth (cf. Low German Tähn, Dutch tand, German Zahn).
Germanic *anþara- becomes English other, West Frisian oar, East Frisian uur, Old Saxon āthar (cf. German & Dutch ander- [þ→d]).
Germanic *fimf becomes English five, West Frisian fiif, East Frisian fieuw, Dutch vijf, Low German fiev, fief (cf. German fünf).
Germanic *samft- becomes English soft, West Frisian sêft, Low German sacht, Dutch zacht [ft→xt] (cf. German sanft).
Germanic *gans- becomes English goose, West Frisian goes, Low German Goos (cf. Dutch gans, German Gans).

Dutch

Dutch displays the spirant shift inconsistently, following the law for some words but not others; this must be understood in terms of the standard language based on the variety (varieties) used in the larger towns of southern Holland, which lost most but not all of its Ingvaeonic characteristics under the influence of the Brabant variety:

(Original) Met uitzondering van brocht -> bracht kan mogelijke invloed van de noordoostelijke dialecten hier niet ingeroepen worden, want die vertoonden ook vrij veel ingweoonse trekken. Gedacht dient te worden aan een gebied zonder ingweoonse kenmerken en in het licht van de immigratiestromen in die tijd ligt dan veeleer Brabantse invloed voor de hand. (Translation: With the exception of brocht -> bracht [brought], we cannot call upon the northeastern dialects to justify the disappeance of Ingvaeonic characteristics, as they also show quite a lot of Ingvaeonic traits. One must think of another region without Ingvaeonic characteristics, and in the light of the currents of immigration to Holland's big southern cities at that time after the fall of Antwerp in 1585, an influence from Brabant is much more likely."
— Johan Taeldeman, "De opbouw van het AN: meer zuidelijke dan oostelijke impulsen", in Tijdschrift voor de Nederlandse Taal- en Letterkunde, deel 123 (2007), afl. 2, p. 104.

Similarly, certain North German dialects retain Old Saxon forms, with the result that very few words in Modern Standard German (based on High German varieties) display this shift; for example, alongside sanft German also has sacht, both meaning "soft", "gentle".

English

One consequence of this is that English has very few words ending in -nth; those which do exist must have entered the vocabulary subsequent to the productive period of the nasal spirant law:

month - derives from Old English monaþ (cf. German Monat); the intervening vowel rendered the law inapplicable here.
tenth - a neologism in Middle English. The original Germanic *tehunþ- was affected by the law, producing Old English tēoþa (Modern English tithe), but the force of analogy to the cardinal number ten caused Middle English speakers to recreate the regular ordinal and re-insert the nasal consonant.
plinth - a direct Greek loan-word in Modern English (πλίνθος).

References

Bremmer Jr, Rolf H. An Introduction to Old Frisian. History, Grammar, Reader, Glossary. (Amsterdam and Philadelphia: John Benjamins, 2009.)
Markey, Thomas L. Germanic dialect grouping and the position of Ingvæonic.(Inst. f. Sprachwissenschaft d. Univ. Innsbruck, 1976.) ISBN:3851245296