The distribution of the primary Germanic dialect groups in Europe in around AD 1:

  North Germanic

  North Sea Germanic, or Ingvaeonic

  Weser-Rhine Germanic, or Istvaeonic

  Elbe Germanic, or Irminonic

  East Germanic

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. In this sound change, in certain combinations vowel + nasal + fricative, the nasal disappeared, with compensatory lengthening of the vowel. ("Spirant" is an older term for "fricative".) The sequences in question are -ns-, -mf-, and -nþ-. The sequence -nh- had undergone a similar change in late Proto-Germanic several hundred years earlier, so this is not unique to the Ingvaeonic languages.

  Examples

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

Gothic uns Old High German uns Middle Dutch ons

Old English ūs Old Frisian ūs West Flemish uus Limburgian oos, ós Old Saxon ūs

Gothic represents East Germanic, and its correspondence to German and Standard Dutch shows it retains the more conservative form. The /n/ has disappeared in English, Frisian, Low German, and dialectal Dutch with compensatory lengthening of the /u/. This phenomenon is therefore observable throughout the "Ingvaeonic" languages. It does not affect High German, East Germanic or North Germanic. The absence of a nasal in Old Norse oss is a separate development, as can be seen by the lack of compensatory lengthening of the vowel.

Likewise:

• Germanic *tanþs → English tooth, North Frisian tôs, toss (vs. Low German Tähn, Dutch and Swedish tand, German Zahn, Icelandic tönn).
• Germanic *anþeraz → English other, Icelandic aðrir, West Frisian oar, West Flemish (Frans-Vlaams) aajer, Old Saxon ōðar, āthar (vs. Low German anner, German/Dutch ander [þ→d], Icelandic: annað/annar/önnur, Swedish annat/annan/andre/andra).
• Germanic *gans → English goose, West Frisian goes, guos, Low German Goos, Faroese gás, Swedish gås (vs. Dutch gans, German Gans).
• Germanic *fimf → English five, West Frisian fiif, East Frisian fieuw, Dutch vijf, Low German fiev, fief (vs. German fünf, Icelandic fimm, Swedish fem).
• Germanic *samftō, -ijaz → English soft, West Frisian sêft, Low German sacht, Dutch zacht [ft→xt] (vs. German sanft).

  English

English shows the results of the shift consistently throughout its repertoire of native lexemes. One consequence of this is that English has very few words ending in -nth; those that 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 - from Middle English tenthe. The original Germanic *tehundô, which was regularised to *tehunþô in early Ingvaeonic, was affected by the law, producing Old English teogoþa, tēoþa (Modern English tithe). But the force of analogy with the cardinal number ten caused Middle English speakers to recreate the regular ordinal and re-insert the nasal consonant.
• plinth - a loanword in Modern English from Greek (πλίνθος "brick, tile").

Likewise, the rare occurrences of the combinations -nf-, -mf- and -ns- have similar explanations.

• answer - originally had an intervening dental: Old English andswaru.
• unfair - the prefix un- is still productive.

  Dutch

Coastal dialects of Dutch apply the spirant law to some extent, shifting some words but not others, e.g. standard Dutch mond 'mouth' vs. Holland mui (earlier muide) 'slit between sandbanks where tidal streams flow into', but standard Dutch tends not to show a shift: although it is based mostly on the coastal dialect of South Holland, which in turn was influenced by Frisian, it was also heavily influenced by the unshifted Brabantine dialect:

(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) "Except for brocht → bracht "brought", the possible influence of the northeastern dialects [Low German] cannot be cited as evidence, since they also show quite a lot of ingvaeonic traits. One must instead think of a region without ingvaeonic traits, and given the direction of immigration of that time [into Holland's larger southern cities following the fall of Antwerp in 1585], Brabantine influence is a straightforward explanation."

—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.

  German

Modern Standard German is based on High German varieties, which are not affected by the shift, but contains some words from North German dialects that do reflect it. So for example, alongside sanft German also has sacht, both meaning "soft", "gentle".

  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 3-85124-529-6