Broadly speaking, the changes that characterise the Ingvaeonic languages can be divided into two groups, those being changes that occurred after the split from Proto-Northwest-Germanic and those preceding it.^[3] Linguistic evidence for changes after the split from Proto-Northwest-Germanic are observed in Old Frisian, Old English and Old Saxon is as follows:

• The so-called Ingvaeonic nasal spirant law: converted *munþ "mouth" into *mų̄þ (compare Old English mūþ).^[4]
• Loss of the third-person reflexive pronouns^[5]
• The loss of person distinctions in plural forms of verbs, which reduced three forms into one form:^[6] merged *habjum "we have" and *habēþ "you (plural) have" with *habją̄þ "they have"
• Palatalisation of velar consonants before front vowels; while the Anglo-Frisian languages further develop these palatal consonants into continuants as in church.^[7]
• Lack of i-mutation in s/z-stem plurals; compare Anglian OE lombur "lambs" with OHG lembir^[8]
• The development of Class III weak verbs into a relic class consisting of four verbs (*sagjan "to say", *hugjan "to think", *habjan "to have", *libjan "to live")
• The split of the Class II weak verb ending*-ōn into *-ōjan: converted *makōn "to make" into *makōjan^[9]
• Development of a plural ending *-ōs in a-stem nouns.^[10]
• Development of numerous new words, such as the replacement of *newun "nine" with *nigun and *minni "less" (adverb) with *laisi^[11]

Changes originating in Proto-Northwest-Germaic, like Old Norse but unlike Gothic and Old High German, include:^[12]

• Dative plurals and first person plural forms in numerous paradigms reduced to -um/-un. Compare an-stem dative plural han-ōm/ōn (OHG) and han-am (Gothic) with hǫn-um (ON), han-um/un (OS) and han-um (OE).
• Elimination of the weak stem -in- in n-stem noun paradigms. For example, OHG gen/dat. sg. han-en and Gothic han-in(s) versus OE han-an, OS han-an/on, OF hon-a, and ON han-a.
• Shortening of pronominal and adjectival non-feminine dative singulars like ON þeim, OE þǣm~þām, OF thām, and OS thēm, all of which have eliminated the final vowel; contrast Gothic þamma as well as OHG dëmu, dëmo, thëmu, thëmo and the like.

While a majority of scholars count Low German as part of North Sea Germanic, others dispute its membership. It shows a mixture of North Sea Germanic and non-North Sea Germanic features. At least at least some of this mixture comes from early and pervasive influence from High German dialects, probably beginning around 700 CE.^[13] Low German also shares a number of features with Low Franconian dialects (Dutch) that are not shared by Anglo-Frisian.^[14]

Old Saxon consistently shows the Ingvaeonic nasal spirant law.^[15] However, Middle Low German dialects restore many nasal consonants lost through the spirant law, giving forms such as ander rather than Old Saxon othar ("other"). In some words, the presence or absence of the nasal fluctuates by dialect, with western dialects using us ("us") while eastern dialects use uns. Some of these changes may be due to leveling of forms with and without the nasal, while others point to High German influence. High German influence on Low Saxon vocabulary is already visible in the Old Saxon period, as Old Saxon attests words such as kind and urkundeo that do not follow the nasal spirant law.^[16][17][a]

In Old and Middle Saxon, palatalized forms of /k/ and /ɡ/ (=[ɣ] in most positions) are common, with palatalized /k/ indicated in the orthography by ⟨ki⟩ (e.g. kiennen, cf. High German kennen) or in some cases by <z> (e.g. zind, cf. High German Kind), while palatalized /g/ was indicated by <i(j)> or sometimes <gi> (e.g. ielden, cf. High German gelten).^[19] The palatalization of /k/ probably occurred over a wide area and to differing amounts in different dialects; in modern Low German, it has in most but not all cases been reversed to /k/.^[20] Outside of many place names, one modern survival is the word sever ("beetle"), still used in many Low German dialects and equivalent to High German Käfer.^[21] Earlier /ɡ/, on the other hand, often alternates with /j/ or is a palatal fricative in modern Low German German dialects, often including in the environment of back vowels.^[22]

The fronting of *a is found consistently in Old English and Old Frisian (e.g. Old English dæg vs. Old High German tac "day"), but took place only partially in Low Saxon, producing doublets of words with a/e in Old Saxon. In Middle Low German, most of these doublets were eliminated in favor of the a version: of the Old Saxon variants glas and gles only glas is found in Middle Low German.^[15][23]

In Anglo-Frisian, a was often rounded to o in front of a nasal consonant, e.g. Old English mon "man". Although this change is attested in Old Saxon with forms such as hond "hand", in Middle Low German forms with a have come to dominate. However, in cases where the nasal consonant has been lost before a spirant and the o lengthened, the o vowel remained: gōs "goose" (e.g. modern Eastphalian gous).^[24][25][b] Forms with o are also found in Western Dutch dialects of Hollandic, Flemish, and Zealandic in some cases, e.g. sochte "soft" in medieval Flemish (standard Dutch zacht). These forms appear connected to the related change in Anglo-Frisian and Old Saxon.^[27]

Lastly, metathesis of vowel sequences and /r/ has traditionally been considered a North Sea Germanic trait. It is commonly found in Frisian but less commonly so in English, and is also present in Low German. Metathesis of r clusters is attested in Old Saxon from the 9th century onward, when a Westphalian manuscript attests hers "horse" (cf. High German Ross); however, it is possible that this form is a loanword from Frisian.^[28] Other infrequent cases of metathesis have been argued to exist from then onward, especially in forms of names ending in -berht;^[29] metathesized forms of the word for "horse" are found in three of the four attested Old Saxon dialects (the fourth does not attest the word), with Westphalian showing a mix of metathesized and non-metathesized forms.^[30] Arjen Versloot and Elżbieta Adamczyk argue that metathesis is a common enough linguistic process that it is not a useful diagnostic for Old Saxon's membership in North Sea Germanic.^[31]

Within Dutch (Low Franconian) dialects, Ingvaeonisms can be further divided into older Ingvaeonisms, which are found through Low Franconian, and younger Ingvaeonisms, which are only found in the coastal areas.^[32] Low Franconian shows cases of the nasal spirant law through its whole dialect area, such as vijf ("five" cf. High German fünf), whereas others are restricted to coastal dialects, such as mui(den), used for river mouths in place names and cognate with standard Dutch mond "mouth".^[32] r-metathesis is also common in Dutch; however, it appears to have a different origin than the r-metathesis in Old Saxon or Anglo-Frisian.^[33] Forms where a has rounded to o before a nasal also found in Western Dutch dialects of Hollandic, Flemish, and Zealandic in some cases, e.g. sochte "soft" in medieval Flemish (standard Dutch zacht). These forms appear connected to the related change in Anglo-Frisian and Old Saxon.^[27]

• Van Bree, Cor (2013). "The spectrum of spatial varieties of Dutch: The historical genesis". In Frans Hinskens; Johan Taeldeman (eds.). Dutch. Language and Space: An International Handbook of Linguistic Variation, Volume 3. Berlin: De Gruyter Mouton. pp. 100–128. doi:10.1515/9783110261332.81.
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• Gallée, Johan Hendrik (1993). Altsächsische Grammatik. Mit Berichtigungen und Literaturnachträgen. Nach Wendelin Försters letzter Ausgabe in Auswahl bearbeitet und mit Einleitung und Glossar versehen (3 ed.). Niemeyer. doi:10.1515/9783110920147.
• Krogmann, Willy (1970). "ALTSÄCHSISCH UND MITTELNIEDERDEUTSCH". In Schmitt, Ludwig E. (ed.). Kurzer Grundriß der germanischen Philologie bis 1500. Band 1: Sprachgeschichte. de Gruyter. pp. 211–252. doi:10.1515/9783110822717.211.
• Harbert, Wayne (2006). The Germanic Languages. Cambridge University Press. ISBN 978-0-521-80825-5.
• Lasch, Agathe (1974). Mittelniederdeutsche Grammatik (2 ed.). Niemeyer. doi:10.1515/9783111393124.
• van Loon, Jozef (2003). "De Chronologie van de R-Metathesis In Het Nederlands En Aangranzende Germaanse Talen". Amsterdamer Beiträge zur älteren Germanistik. 57 (1): 141–167. doi:10.1163/18756719-90000136.
• Ringe, Donald; Taylor, Ann (2014). The Development of Old English – A Linguistic History of English, vol. II. United States of America: Oxford University Press.
• Rübekeil, Ludwig (2017). "The dialectology of Germanic". Handbook of Comparative and Historical Indo-European Linguistics. Vol. 2. de Gruyter Mouton. pp. 986–1002. doi:10.1515/9783110523874-013.
• Stiles, Patrick V. (2013-01-01). "The Pan-West Germanic Isoglosses and the Subrelationships of West Germanic to Other Branches". NOWELE: North-Western European Language Evolution. 66 (1): 5–38. doi:10.1075/nowele.66.1.02sti. ISSN 0108-8416.
• de Vaan, Michiel (2017). The Dawn of Dutch: Language contact in the Western Low Countries before 1200. John Benjamins. doi:10.1075/nss.30.
• Versloot, Arjen; Adamczyk, Elżbieta (2017). "The Geography and Dialects of Old Saxon: River-basin communication networks and the distributional patterns of North Sea Germanic features in Old Saxon". In Hines, John; IJssennagger-van der Pluijm, Nelleke (eds.). Frisians and their North Sea Neighbours: From the Fifth Century to the Viking Age. Boydell and Brewer. pp. 125–148. doi:10.1515/9781787440630-014.
• Zhirmunski, Viktor M. (2010). Naidich, Larissa (ed.). Deutsche Mundartkunde. Peter Lang.