The sound change affected sequences of vowel + nasal consonant + fricative consonant. The sequences in question are -ns-, -mf-, and -nþ-, preceded by any vowel. The nasal consonant disappeared, sometimes causing nasalization and compensatory lengthening of the vowel before it. The nasalization disappeared relatively soon after in many dialects along the coast, but it was retained long enough to prevent Anglo-Frisian brightening of to. The resulting long nasalized vowel was rounded to in most languages under various circumstances. In Old Saxon on the other hand, the nasal consonant is later restored in all but a small handful of forms, so that Old Saxon appears as in all Middle Low German dialects, while Old Saxon appears as in all Middle Low German dialects. The Old Saxon words and appear variably with and without a restored consonant, an example being the combination of and on the Baltic coast. The sequence -nh- had already undergone a similar change in late Proto-Germanic several hundred years earlier, and affected all Germanic languages, not only the Ingvaeonic subgroup. The result of this earlier change was the same: a long nasal vowel. However, the nasalization in this earlier case did not cause rounding of nasal in Old Saxon, which instead became simple, while the later Ingvaeonic spirant law resulted in. In Old English and Old Frisian, rounding occurred here as well, giving in both cases. It was this earlier shift that created the n/∅ in think/thought and bring/brought.
Examples
Compare the first person plural pronoun "us" in various old Germanic languages: 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. Likewise:
Germanic *tanþs > English tooth, North Frisiantôs, toss.
Germanic *anþeraz > English other, Icelandic aðrir, West Frisianoar, West Flemish aajer, Old Saxon ōðar, āthar.
Germanic *gans > English goose, West Frisian goes, guos, Low German Goos.
Germanic *fimf > English five, West Frisian fiif, East Frisianfieuw, Dutch vijf, Low German fiev, fief.
Germanic *samftō, -ijaz > English soft, West Frisian sêft, Low German sacht, Dutch zacht .
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 ; 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. But the force of analogy with the cardinal numberten caused Middle English speakers to recreate the regular ordinal and re-insert the nasal consonant.
Likewise, the rare occurrences of the combinations -nf-, -mf- and -ns- have similar explanations.
answer - originally had an intervening stop: Old English andswaru.
unfair - the prefix un- is still productive.
Dutch
Although Dutch is based mostly on the Hollandic dialects, which in turn were influenced by Frisian, it was also heavily influenced by the Brabantian dialect which tends not to show a shift. As a result, the shift is generally not applied but is still applied to some words. For example Dutch vijf vs. German fünf, zacht vs. sanft. Coastal dialects of Dutch tend to have more examples, e.g. standard Dutch mond "mouth" vs. Hollandic mui "slit between sandbanks where tidal streams flow into". Brabantian dialects tend to have fewer examples, having unshifted examples in a few cases where standard Dutch has the shift, as in the toponym Zonderwijk which is cognate to standard Dutch zuid "south".
German
The spirant law was originally active in Central Franconian dialects of High German, a proof that it was not entirely restricted to Ingvaeonic. Compare for example Luxembourgisheis, Gaus. Modern Standard German is based more on eastern varieties which are not affected by the shift. The standard language does, however, contain a number of Low German borrowings with it. For example Süden, or sacht.
