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-.
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:
Note that Dutch is inconsistent, following the law in some words but not others; this must be understood in terms of the standard language drawing from a variety of dialects, only some of which were affected by the sound change. Similarly, certain North German dialects retain Old Saxon forms, with the result that a very few words in Modern Standard German have this shift: alongside sanft German also has sacht, both meaning "soft", "gentle".
One consequence of this is that English has very few words ending in -nth; those which do exist must be more recent than the productive period of the Anglo-Frisian nasal spirant law:

Old English ūs
Old Frisian ūs
Old Saxon ūs
Old High German uns
Middle Dutch ons
Gothic uns
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).
month - in Old English this was monaþ (cf. German Monat); the intervening vowel made the law inoperable.
tenth - a neologism in Middle English. Germanic *tehunþ- did originally follow the law, producing Old English tēoþa (Modern English tithe), but the force of analogy to the cardinal number ten caused Middle English to recreate the regular ordinal.
plinth - a Greek loan-word in Modern English (πλίνθος).