Editing Germanic umlaut (section)

== Historical survey by language ==

===West Germanic languages===

Although umlauts operated the same way in all the West Germanic languages, the exact words in which it took place and the outcomes of the process differ between the languages. Of particular note is the loss of word-final {{lang|gem-x-proto|-i}} after heavy syllables. In the more southern languages (Old High German, Old Dutch, Old Saxon), forms that lost {{lang|gem-x-proto|-i}} often show no umlaut, but in the more northern languages (Old English, Old Frisian), the forms do. Compare Old English {{lang|ang|ġiest}} "guest", which shows umlaut, and Old High German {{lang|goh|gast}}, which does not, both from Proto-Germanic {{lang|gem-x-proto|*gastiz}}. That may mean that there was dialectal variation in the timing and spread of the two changes, with final loss happening before umlaut in the south but after umlaut in the north. On the other hand, umlaut may have still been partly allophonic, and the loss of the conditioning sound may have triggered an "un-umlauting" of the preceding vowel. Nevertheless, medial {{lang|gem-x-proto|-ij-}} consistently triggers umlaut although its subsequent loss is universal in West Germanic except for Old Saxon and early Old High German.

====I-mutation in Old English====
[[File:Vowels of proto-Old English and general change when i-mutated.png|thumb|The vowels and diphthongs of proto-Old English prior to i-mutation (in black) and how they generally changed under i-mutation (in red). Outcomes varied according to dialect; i-mutation of diphthongs is given for Early West Saxon as spelled in manuscripts due to uncertainty about the precise phonetic value of the graph.]]
I-mutation generally affected Old English vowels as follows in each of the main dialects.{{sfnp |Campbell |1959 |pp=112, 190–204, 288}} It led to the introduction into Old English of the new sounds {{IPA|/y(ː)/}}, {{IPA|/ø(ː)/}} (which, in most varieties, soon turned into {{IPA|/e(ː)/}}), and a sound written in Early West Saxon manuscripts as {{angbr|ie}} but whose phonetic value is debated.
{| class="wikitable"
|+ i-mutation
|-
!rowspan=2| Original
!colspan=3| i-mutated
!rowspan=2| Examples and notes
|-
!West Saxon
!Anglian
!Kentish
|-
| {{lang|ang|a}} || colspan="2" | {{lang|ang|æ, e}}
| {{lang|ang|æ, e}} > {{lang|ang|e}} || {{lang|ang|bacan}} "to bake", {{lang|ang|bæcþ}} "(he/she) bakes". {{lang|ang|a}} > {{lang|ang|e}} particularly before nasal consonants: {{lang|ang|mann}} "person", {{lang|ang|menn}} "people"
|-
| {{lang|ang|ā}} || colspan="3" | {{lang|ang|ǣ}}
|| {{lang|ang|lār}} "teaching" (cf. "lore"), {{lang|ang|lǣran}} "to teach"
|-
| {{lang|ang|æ}}
| colspan="3"| {{lang|ang|e}} || {{lang|ang|þæc}} "covering" (cf. "thatch"), {{lang|ang|þeccan}} "to cover"
|-
| {{lang|ang|e}} || colspan="3" | {{lang|ang|i}} || not clearly attested due to earlier Germanic {{lang|gem-x-proto|e}} > {{lang|gem-x-proto|i}} before {{lang|gem-x-proto|i}}, {{lang|gem-x-proto|j}}
|-
| {{lang|ang|o}} || colspan="3" | {{lang|ang|ø}} > {{lang|ang|e}}
|| Latin {{lang|la|olium}}, Old English {{lang|ang|øle}} > {{lang|ang|ele}}.
|-
| {{lang|ang|ō}}
| colspan="3" | {{lang|ang|ø̄}} > {{lang|ang|ē}}
| {{lang|ang|fōt}} "foot", {{lang|ang|fø̄t}} > {{lang|ang|fēt}} "feet".
|-
| {{lang|ang|u}} || colspan="2" | {{lang|ang|y}}
| {{lang|ang|y}} > {{lang|ang|e}} || {{lang|ang|murnan}} "to mourn", {{lang|ang|myrnþ}} "(he/she) mourns"
|-
| {{lang|ang|ū}}
| colspan="2"| {{lang|ang|ȳ}}
| {{lang|ang|ȳ}} > {{lang|ang|ē}} || {{lang|ang|mūs}} "mouse", {{lang|ang|mȳs}} "mice"
|-
| {{lang|ang|ea}} || {{lang|ang|ie}} > {{lang|ang|y}}
| colspan="2" | {{lang|ang|e}} || {{lang|ang|eald}} "old", {{lang|ang|ieldra}}, {{lang|ang|eldra}} "older" (cf. "elder")
|-
| {{lang|ang|ēa}}
|| {{lang|ang|īe}} > {{lang|ang|ȳ}}
| colspan="2" | {{lang|ang|ē}} || {{lang|ang|nēah}} "near" (cf. "nigh"), {{lang|ang|nīehst}} "nearest" (cf. "next")
|-
| {{lang|ang|eo}} || colspan="3" | {{lang|ang|io}} > {{lang|ang|eo}} || examples are rare due to earlier Germanic {{lang|gem-x-proto|e}} > {{lang|gem-x-proto|i}} before {{lang|gem-x-proto|i}}, {{lang|gem-x-proto|j}}. {{lang|ang|io}} became {{lang|ang|eo}} in most later varieties of Old English
|-
| {{lang|ang|ēo}} || colspan="3" | {{lang|ang|īo}} > {{lang|ang|ēo}}
|| examples are rare due to earlier Germanic {{lang|gem-x-proto|e}} > {{lang|gem-x-proto|i}} before {{lang|gem-x-proto|i}}, {{lang|gem-x-proto|j}}. {{lang|ang|īo}} became {{lang|ang|ēo}} in most later varieties of Old English
|-
| {{lang|ang|io}}
| {{lang|ang|ie}} > {{lang|ang|y}}
| colspan="2" | {{lang|ang|io}}, {{lang|ang|eo}}
| {{lang|ang|*fiohtan}} "to fight", {{lang|ang|fieht}} "(he/she) fights". {{lang|ang|io}} became {{lang|ang|eo}} in most later varieties of Old English, giving alternations like {{lang|ang|beornan}} "to burn", {{lang|ang|biernþ}} "(he/she) burns"
|-
| {{lang|ang|īo}} || {{lang|ang|īe}} > {{lang|ang|ȳ}}
| colspan="2" | {{lang|ang|īo}}, {{lang|ang|ēo}} || {{lang|ang|līoht}} "light", {{lang|ang|līehtan}} "illuminate". {{lang|ang|īo}} became {{lang|ang|ēo}} in most later varieties of Old English, giving alternations like {{lang|ang|sēoþan}} "to boil" (cf. "seethe"), {{lang|ang|sīeþþ}} "(he/she) boils"
|}

I-mutation is particularly visible in the inflectional and derivational [[Old English morphology|morphology of Old English]] since it affected so many of the Old English vowels. Of 16 basic vowels and diphthongs in [[Old English]], only the four vowels {{lang|ang|ǣ, ē, i, ī}} were unaffected by i-mutation. Although i-mutation was originally triggered by an {{IPA|/i(ː)/}} or {{IPA|/j/}} in the syllable following the affected vowel, by the time of the surviving Old English texts, the {{IPA|/i(ː)/}} or {{IPA|/j/}} had generally changed (usually to {{IPA|/e/}}) or been lost entirely, with the result that i-mutation generally appears as a morphological process that affects a certain (seemingly arbitrary) set of forms. These are most common forms affected:
*The plural, and genitive/dative singular, forms of consonant-declension nouns (Proto-Germanic (PGmc) {{lang|gem-x-proto|-iz}}), as compared to the nominative/accusative singular &ndash; e.g., {{lang|ang|fōt}} "foot," {{lang|ang|fēt}} "feet;" {{lang|ang|mūs}} "mouse," {{lang|ang|mȳs}} "mice." Many more words were affected by this change in Old English versus modern English, for example, {{lang|ang|bōc}} "book," {{lang|ang|bēċ}} "books;" {{lang|ang|frēond}} "friend," {{lang|ang|frīend}} "friends."
*The second and third person [[present tense|present]] [[Grammatical number|singular]] [[indicative mood|indicative]] of [[Germanic strong verb|strong verb]]s (Pre-Old-English (Pre-OE) {{lang|mis|*-ist}}, {{lang|mis|*-iþ}}), as compared to the [[infinitive]] and other present-tense forms &ndash; e.g. {{lang|ang|helpan}} "to help," {{lang|ang|helpe}} "(I) help," {{lang|ang|hilpst}} "(you sg.) help," {{lang|ang|hilpþ}} "(he/she) helps," {{lang|ang|helpaþ}} "(we/you pl./they) help."
*The [[comparative]] form of some [[adjective]]s (Pre-OE {{lang|mis|*-ira}} < PGmc {{lang|gem-x-proto|-izǭ}}, Pre-OE {{lang|mis|*-ist}} < PGmc {{lang|gem-x-proto|-istaz}}), as compared to the base form &ndash; e.g. {{lang|ang|eald}} "old," {{lang|ang|ieldra}} "older," {{lang|ang|ieldest}} "oldest" (cf. "elder, eldest").
*Throughout the first class of weak verbs (original suffix {{lang|mis|-jan}}), as compared to the forms from which the verbs were derived &ndash; e.g. {{lang|ang|fōda}} "food," {{lang|ang|fēdan}} "to feed" < Pre-OE {{lang|mis|*fōdjan}}; {{lang|ang|lār}} "lore," {{lang|ang|lǣran}} "to teach;" {{lang|ang|feallan}} "to fall," {{lang|ang|fiellan}} "to fell."
*In the abstract nouns in {{lang|ang|þ(u)}} (PGmc {{lang|gem-x-proto|-iþō}}) corresponding to certain adjectives &ndash; e.g., {{lang|ang|strang}} "strong," {{lang|ang|strengþ(u)}} "strength;" {{lang|ang|hāl}} "whole/hale," {{lang|ang|hǣlþ(u)}} "health;" {{lang|ang|fūl}} "foul," {{lang|ang|fȳlþ(u)}} "filth."
*In female forms of several nouns with the suffix {{lang|ang|-enn}} (PGmc {{lang|gem-x-proto|-injō}}) &ndash; e.g., {{lang|ang|god}} "god," {{lang|ang|gydenn}} "goddess" (cf. German {{lang|de|Gott}}, {{lang|de|Göttin}}); {{lang|ang|fox}} "fox," {{lang|ang|fyxenn}} "vixen."
*In i-stem abstract nouns derived from verbs (PGmc {{lang|gem-x-proto|-iz}}) &ndash; e.g. {{lang|ang|cyme}} "a coming," {{lang|ang|cuman}} "to come;" {{lang|ang|byre}} "a son (orig., a being born)," {{lang|ang|beran}} "to bear;" {{lang|ang|fiell}} "a falling," {{lang|ang|feallan}} "to fall;" {{lang|ang|bend}} "a bond," {{lang|ang|bindan}} "to bind." Note that in some cases the abstract noun has a different vowel than the corresponding verb, due to Proto-Indo-European [[ablaut]].

=====Notes=====
#The phonologically expected umlaut of {{IPA|/a/}} is {{IPA|/æ/}}. However, in many cases {{IPA|/e/}} appears. Most {{IPA|/a/}} in Old English stem from earlier {{IPA|/æ/}} because of a change called [[a-restoration]]. This change was blocked when {{IPA|/i/}} or {{IPA|/j/}} followed, leaving {{IPA|/æ/}}, which subsequently mutated to {{IPA|/e/}}. For example, in the case of {{lang|ang|talu}} "tale" vs. {{lang|ang|tellan}} "to tell," the forms at one point in the early history of Old English were {{lang|ang|*tælu}} and {{lang|ang|*tælljan}}, respectively. A-restoration converted {{lang|ang|*tælu}} to {{lang|ang|talu}}, but left {{lang|ang|*tælljan}} alone, and it subsequently evolved to {{lang|ang|tellan}} by i-mutation. The same process "should" have led to {{lang|ang|*becþ}} instead of {{lang|ang|bæcþ}}. That is, the early forms were {{lang|ang|*bæcan}} and {{lang|ang|*bæciþ}}. A-restoration converted {{lang|ang|*bæcan}} to {{lang|ang|bacan}} but left alone {{lang|ang|*bæciþ}}, which would normally have evolved by umlaut to {{lang|ang|*becþ}}. In this case, however, once a-restoration took effect, {{lang|ang|*bæciþ}} was modified to {{lang|ang|*baciþ}} by analogy with {{lang|ang|bacan}}, and then later umlauted to {{lang|ang|bæcþ}}.
#A similar process resulted in the umlaut of {{IPA|/o/}} sometimes appearing as {{IPA|/e/}} and sometimes (usually, in fact) as {{IPA|/y/}}. In Old English, {{IPA|/o/}} generally stems from [[a-mutation]] of original {{IPA|/u/}}. A-mutation of {{IPA|/u/}} was blocked by a following {{IPA|/i/}} or {{IPA|/j/}}, which later triggered umlaut of the {{IPA|/u/}} to {{IPA|/y/}}, the reason for alternations between {{IPA|/o/}} and {{IPA|/y/}} being common. Umlaut of {{IPA|/o/}} to {{IPA|/e/}} occurs only when an original {{IPA|/u/}} was modified to {{IPA|/o/}} by analogy before umlaut took place. For example, {{lang|ang|dohtor}} comes from late [[Proto-Germanic]] {{lang|gem-x-proto|dohter}}, from earlier {{lang|gem-x-proto|duhter}}. The plural in Proto-Germanic was {{lang|gem-x-proto|duhtriz}}, with {{IPA|/u/}} unaffected by a-mutation due to the following {{IPA|/i/}}. At some point prior to i-mutation, the form {{lang|gem-x-proto|duhtriz}} was modified to {{lang|gem-x-proto|dohtriz}} by analogy with the singular form, which then allowed it to be umlauted to a form that resulted in {{lang|ang|dehter}}.

A few hundred years after i-umlaut began, another similar change called double umlaut occurred. It was triggered by an {{IPA|/i/}} or {{IPA|/j/}} in the third or fourth syllable of a word and mutated ''all'' previous vowels but worked only when the vowel directly preceding the {{IPA|/i/}} or {{IPA|/j/}} was {{IPA|/u/}}. This {{IPA|/u/}} typically appears as {{angbr|e}} in Old English or is deleted:

* {{lang|ang|hægtess}} "witch" < PGmc {{lang|gem-x-proto|hagatusjō}} (cf. [[Old High German]] {{lang|goh|hagazussa}})
* {{lang|ang|ǣmerge}} "embers" < Pre-OE {{lang|mis|*āmurja}} < PGmc {{lang|gem-x-proto|aimurjǭ}} (cf. Old High German {{lang|goh|eimurja}})
* {{lang|ang|ǣrende}} "errand" < PGmc {{lang|gem-x-proto|ǣrundijaz}} (cf. [[Old Saxon]] {{lang|osx|ārundi}})
* {{lang|ang|efstan}} "to hasten" < archaic {{lang|ang|øfestan}} < Pre-OE {{lang|mis|*ofustan}}
* {{lang|ang|ȳmest}} "upmost" < PGmc {{lang|gem-x-proto|uhumistaz}} (cf. [[Gothic language|Gothic]] {{lang|got-Latn|áuhumists}})

As shown by the examples, affected words typically had {{IPA|/u/}} in the second syllable and {{IPA|/a/}} in the first syllable. The {{IPA|/æ/}} developed too late to break to {{lang|ang|ea}} or to trigger palatalization of a preceding velar.

==== I-mutation in High German ====
I-mutation is visible in [[Old High German]] (OHG), c. 800 CE, only on short {{IPA|/a/}}, which was mutated to {{IPA|/e/}} (the so-called "primary umlaut"), although in certain phonological environments the mutation fails to occur. By then, it had already become partly phonologized, since some of the conditioning {{IPA|/i/}} and {{IPA|/j/}} sounds had been deleted or modified. The later history of German, however, shows that {{IPA|/o/}} and {{IPA|/u/}}, as well as long vowels and diphthongs, and the remaining instances of {{IPA|/a/}} that had not been umlauted already, were also affected (the so-called "secondary umlaut"); starting in Middle High German, the remaining conditioning environments disappear and {{IPA|/o/}} and {{IPA|/u/}} appear as {{IPA|/ø/}} and {{IPA|/y/}} in the appropriate environments.

That has led to a controversy over when and how i-mutation appeared on these vowels. Some (for example, Herbert Penzl){{sfnp |Penzl |1949}} have suggested that the vowels must have been modified without being indicated for lack of proper symbols and/or because the difference was still partly allophonic. Others (such as Joseph Voyles){{sfnp |Voyles |1992}} have suggested that the i-mutation of {{IPA|/o/}} and {{IPA|/u/}} was entirely analogical and pointed to the lack of i-mutation of these vowels in certain places where it would be expected, in contrast to the consistent mutation of {{IPA|/a/}}. Perhaps{{Original research inline|date=February 2010}} the answer is somewhere in between &mdash; i-mutation of {{IPA|/o/}} and {{IPA|/u/}} was indeed phonetic, occurring late in OHG, but later spread analogically to the environments where the conditioning had already disappeared by OHG (this is where failure of i-mutation is most likely).{{Citation needed|date=February 2010}} It must also be kept in mind that it is an issue of relative chronology: already early in the history of attested OHG, some umlauting factors are known to have disappeared (such as word-internal {{IPA|/j/}} after geminates and clusters), and depending on the age of OHG umlaut, that could explain some cases where expected umlaut is missing. The whole question should now be reconsidered in the light of [[Fausto Cercignani]]'s suggestion that the Old High German umlaut phenomena produced phonemic changes before the factors that triggered them off changed or disappeared, because the umlaut allophones gradually shifted to such a degree that they became distinctive in the phonological system of the language and contrastive at a lexical level.{{sfnp |Cercignani |2022a}}

However, sporadic place-name attestations demonstrate the presence of the secondary umlaut already for the early 9th century, which makes it likely that all types of umlaut were indeed already present in Old High German, even if they were not indicated in the spelling. Presumably, they arose already in the early 8th century.{{sfnp |Gütter |2011 }} [[Ottar Grønvik]], also in view of spellings of the type {{angbr|ei}}, {{angbr|ui}}, and {{angbr|oi}} in the early attestations, affirms the old [[epenthesis]] theory, which views the origin of the umlaut vowels in the insertion of {{IPA|/j/}} after back vowels, not only in West, but also in North Germanic.{{sfnp |Grønvik |1998}} [[Fausto Cercignani]] prefers the assimilation theory and presents a history of the OHG umlauted vowels up to the present day.{{sfnp |Cercignani |2022b }}

In modern German, umlaut as a marker of the plural of nouns is a regular feature of the language, and although umlaut generally is no longer a productive force in German, new plurals of this type can be created by analogy. Likewise, umlaut marks the comparative of many adjectives and other kinds of inflected and derived forms. Borrowed words have acquired umlaut as in {{lang|de|Chöre}} 'choirs' or {{lang|de|europäisch}} 'European.' Umlaut seems to be totally productive in connection with diminutive suffix {{lang|de|-chen}}, as in {{lang|de|Skandäl-chen}} 'little scandal.' 

Because of the grammatical importance of such pairs, the German [[umlaut (diacritic)|umlaut diacritic]] was developed, making the phenomenon very visible. The result in German is that the vowels written as {{angbr|a}}, {{angbr|o}}, and {{angbr|u}} become {{angbr|ä}}, {{angbr|ö}}, and {{angbr|ü}}, and the diphthong {{angbr|au}} {{IPA|/aʊ/}} becomes {{angbr|äu}} {{IPA|/ɔʏ/}}: {{lang|de|Mann}} {{IPA|de|man|}} "man" vs. {{lang|de|Männer}} {{IPA|de|ˈmɛnɐ|}} "men," {{lang|de|Fuß}} {{IPA|de|fuːs|}} "foot" vs. {{lang|de|Füße}} {{IPA|de|ˈfyːsə|}} "feet," {{lang|de|Maus}} {{IPA|de|maʊs|}} "mouse" vs. {{lang|de|Mäuse}} {{IPA|de|ˈmɔʏzə|}} "mice."

In various dialects, the umlaut became even more important as a morphological marker of the plural after the apocope of final schwa ({{lang|de|-e}}); that rounded front vowels have become unrounded in many dialects does not prevent them from serving as markers of the plural given that they remain distinct from their non-umlauted counterparts (just like in English ''foot'' – ''feet'', ''mouse'' – ''mice''). The example {{lang|de|Gast}} "guest" vs. {{lang|de|Gäst(e)}} "guests" served as the model for analogical pairs like {{lang|de|Tag}} "day" vs. {{lang|de|Täg(e)}} "days" (vs. standard {{lang|de|Tage}}) and {{lang|de|Arm}} "arm" vs. {{lang|de|Ärm(e)}} "arms" (vs. standard {{lang|de|Arme}}). Even plural forms like {{lang|de|Fisch(e)}} "fish," which had never had a front rounded vowel in the first place, were interpreted as such (i.e., as if from Middle High German **{{lang|gmh|füsche}}) and led to singular forms like {{lang|de|Fusch}} {{IPA|de|fʊʃ|}}, which are attested in some dialects.

==== I-mutation in Old Saxon ====
In [[Old Saxon]], umlaut is much less apparent than in Old Norse. The only vowel that is regularly fronted before an {{IPA|/i/}} or {{IPA|/j/}} is short {{IPA|/a/}}: {{lang|osx|gast}} – {{lang|osx|gesti}}, {{lang|osx|slahan}} – {{lang|osx|slehis}}. It must have had a greater effect than the orthography shows since all later dialects have a regular umlaut of both long and short vowels.

==== I-mutation in Dutch ====
Late Old Dutch saw a merger of {{IPA|/u/}} and {{IPA|/o/}}, causing their umlauted results to merge as well, giving {{IPA|/ʏ/}}. The lengthening in open syllables in early Middle Dutch then lengthened and lowered this short {{IPA|/ʏ/}} to long {{IPA|/øː/}} (spelled {{angbr|eu}}) in some words. This is parallel to the lowering of {{IPA|/i/}} in open syllables to {{IPA|/eː/}}, as in {{lang|nl|schip}} ("ship") – {{lang|nl|schepen}} ("ships").

In general, the effects of the Germanic umlaut in plural formation are limited.{{sfnp |Howell |Salmons |1997 |p=93}} One of the defining phonological features of Dutch, is the general absence of the I-mutation or secondary umlaut when dealing with long vowels. Unlike English and German, Dutch does not palatalize the long vowels, which are notably absent from the language.{{sfnp |Willemyns |2013 |p=36}} Thus, for example, where modern German has {{lang|de|fühlen}} {{IPA|/ˈfyːlən/}} and English has ''feel'' {{IPA|/fiːl/}} (from Proto-Germanic {{lang|gem-x-proto|fōlijaną}}), standard Dutch retains a back vowel in the stem in {{lang|nl|voelen}} {{IPA|/ˈvulə(n)/}}. Thus, only two of the original Germanic vowels were affected by umlaut at all in Dutch: {{IPA|/a/}}, which became {{IPA|/ɛ/}}, and {{IPA|/u/}}, which became {{IPA|/ʏ/}} (spelled {{angbr|u}}). As a result of this relatively sparse occurrence of umlaut, standard Dutch does not use umlaut as a grammatical marker. An exception is the noun {{lang|nl|stad}} "city" which has the irregular umlauted plural {{lang|nl|steden}}.

Later developments in Middle Dutch show that long vowels and diphthongs were not affected by umlaut in the more western dialects, including those in western [[Province of Brabant|Brabant]] and [[Holland]] that were most influential for standard Dutch. However in what is traditionally called the ''Cologne Expansion'' (the spread of certain [[Central Franconian languages|West German features]] in the south-easternmost Dutch dialects during the High Medieval period) the more eastern and southeastern dialects of Dutch, including easternmost Brabantian and all of [[Limburgish language|Limburgish]] have umlaut of long vowels (or in case of Limburgish, all rounded back vowels), however.{{sfnp |Belemans |Keulen |2004 |pp=22–25 }} Consequently, these dialects also make grammatical use of umlaut to form plurals and diminutives, much as most other modern Germanic languages do. Compare {{lang|nl|vulen}} {{IPA|/vylə(n)/}} and {{lang|nl|menneke}} "little man" from {{lang|nl|man}}.

===North Germanic languages===
{{Main|Old Norse#Umlaut|l1=Old Norse umlaut}}
{{See also|Old Norse morphology}}Umlaut is a feature of Icelandic, in which both i-umlaut and a-umlaut exist.{{sfnp |Hansson |Wiese |2024 |pp=864–871 }} The situation in Old Norse is complicated as there are two forms of i-mutation. Of these two, only one is phonologized.{{Clarify|reason=Phonemic/etic? (In)effective?|date=August 2010}} I-mutation in Old Norse is phonological:
* In [[Proto-Norse]], if the syllable was [[syllable weight|heavy]] and followed by vocalic {{lang|mis|i}} ({{lang|mis|*gastiʀ}} > {{lang|non|gestr}}, but {{lang|mis|*staði}} > {{lang|non|*stað}}) or, regardless of syllable weight, if followed by consonantal {{lang|mis|i}} ({{lang|mis|*skunja}} > {{lang|non|skyn}}). The rule is not perfect, as some light syllables were still umlauted: {{lang|mis|*kuni}} > {{lang|non|kyn}}, {{lang|mis|*komiʀ}} > {{lang|non|kømr}}.
* In Old Norse, if the following syllable contains a remaining Proto-Norse {{lang|mis|i}}. For example, the root of the dative singular of ''u''-stems are i-mutated as the desinence contains a Proto-Norse {{lang|mis|i}}, but the dative singular of ''a''-stems is not, as their [[desinence]] stems from Proto-Norse {{lang|mis|ē}}.

I-mutation is ''not'' phonological if the vowel of a long syllable is i-mutated by a syncopated ''i''. I-mutation does not occur in short syllables.

{| class="wikitable"
|+ i-mutation
|-
! Original
! Mutated
! Example
|-
| {{lang|non|a}} || {{lang|non|e}} ({{lang|non|ę}}) || {{lang|non|fagr}} (fair) / {{lang|non|fegrstr}} (fairest)
|-
| {{lang|non|au}} || {{lang|non|ey}} || {{lang|non|lauss}} (loose) / {{lang|non|leysa}} (to loosen)
|-
| {{lang|non|á}} || {{lang|non|æ}} || {{lang|non|[[Æsir|Áss / Æsir]]}}
|-
| {{lang|non|o}} || {{lang|non|ø}} || {{lang|non|koma}} (to come) / {{lang|non|kømr}} (comes)
|-
| {{lang|non|ó}} || {{lang|non|œ}} || {{lang|non|róa}} (to row) / {{lang|non|rœr}} (rows)
|-
| {{lang|non|u}} || {{lang|non|y}} || {{lang|non|upp}} (up) / {{lang|non|yppa}} (to lift up)
|-
| {{lang|non|ú}} || rowspan="2" | {{lang|non|ý}} || {{lang|non|fúll}} (foul) / {{lang|non|fýla}} (filth)
|-
| {{lang|non|jú}} || {{lang|non|ljúga}} (to lie) / {{lang|non|lýgr}} (lies)
|-
| {{lang|non|ǫ}} || {{lang|non|ø}} || {{lang|non|sǫkk}} (sank) / {{lang|non|søkkva}} (to sink)
|}