If you have started learning Icelandic nouns, you have already hit this and probably found it baffling: barn means "child," but the plural is börn and the dative plural is börnum. The vowel seems to mutate at random. It does not. This page teaches the single alternation behind it — u-umlaut (u-hljóðvarp) — at the level of sound. The rule is one sentence: a following u rounds a stem a into ö. Once you can see the trigger, the "vowel change" stops being a list of exceptions and becomes a mechanical, predictable rounding you can recognise everywhere — in noun plurals, the dative plural -um, weak verbs, and adjective endings alike.
The rule: u rounds a to ö
The vowel a is an open unrounded vowel — lips relaxed, [a]. The vowel ö is the same height-ish but front and rounded — [œ], lips pushed forward (the page below covers how to make it). When a u sits in the syllable after a stem a, the lips anticipate that rounding and round the a too. The a becomes ö. That is assimilation: one vowel reaching forward to round the one before it.
| Trigger | Base form | U-umlauted form | What changed |
|---|---|---|---|
| plural -u (historic) | barn [a] | börn [œ] | a → ö, the u later lost |
| dative plural -um | dagur [a] | dögum [œ] | a → ö, the u still visible |
| 1pl verb -um | kalla [a] | köllum [œ] | a → ö, the u still visible |
| plural -Ø (historic u) | land [a] | lönd [œ] | a → ö, the u later lost |
barn → börn
child → children — the historic plural ending rounded the a to ö; the u that triggered it has since vanished
dagur → dögum
day → (to) days (dative plural) — the -um ending rounds the stem a; here the trigger u is right there in -um
kalla → köllum
to call → we call — the 1st-person-plural ending -um rounds the stem a to ö, just like in nouns
land → lönd
country → countries — the historic plural u rounded the a, then disappeared, leaving only ö behind
ö and a are the SAME vowel, rounded
The most important thing to internalise: börn is not a different, unrelated word from barn. The vowel is the same vowel quality, just with the lips rounded. Say a [a] — lips neutral. Now, without changing anything else in your mouth, push your lips forward and out into a small "o"-shape and let the vowel front toward land on ö [œ]. That is the entire phonetic difference between barn and börn. The relationship is rounding, not replacement.
This is why treating börn as a vocabulary item to memorise separately from barn is the wrong move — it is the same noun, and the ö is doing predictable grammatical work.
kaldur → köld
cold (masc.) → cold (fem. sg./neut. pl.) — the adjective stem a rounds to ö under the same u-umlaut trigger
tala → tölum
to speak → we speak — stem a rounds to ö before the -um ending; same mechanism as köllum
The trigger is sometimes invisible
Here is the wrinkle that makes u-umlaut feel unpredictable until you know it. Sometimes the triggering u is still sitting right there in the ending, so the cause is obvious:
- dögum — the -um contains the u.
- köllum — the -um contains the u.
But sometimes the u did its rounding work centuries ago and then disappeared, leaving only the ö as a fossil of a u that is no longer written or pronounced:
- börn — the old plural ending was -u (Proto-Norse barnu); the u rounded the a, then eroded away. Modern börn shows only the result.
- lönd — same story: an old u rounded the a, then vanished.
So ö in a paradigm is a clue that a u is (or once was) in the ending — even when you can no longer see it. This is the "chain" the rule describes: a → ö, and the trigger u may survive (dögum) or be lost (börn).
staður → staðir, en stöðum
place → places (nom. pl. staðir, plain a), but dative plural stöðum (a → ö before -um)
gata → götur
street → streets — the plural ending here historically rounded the a to ö before -ur; the result is written ö
hamar → hömrum
hammer → (to) hammers (dative plural) — the -um rounds the stem a; note the second a syncopates away too
How this differs from anything in English
English has nothing remotely like this living, automatic rounding. The closest historical relative is the fossilised umlaut behind irregular plurals like foot → feet, man → men, mouse → mice — and those are the frozen leftovers of the same Germanic umlaut process, now totally unproductive. You cannot predict feet from foot; you just memorise it. Icelandic kept the process alive and regular: the a → ö rounding still fires automatically every time a u lands in the next syllable, across thousands of words. So instead of a short list of irregular plurals, you get one predictable rule that does the work everywhere. The trade is good for the learner: learn the trigger once, and you stop being surprised.
The full declension and conjugation consequences of this alternation recur across the noun, adjective, and verb pages — for the noun plural and dative-plural patterns specifically, see u-Umlaut in Plurals and the Dative Plural. This page's job is just to make the sound alternation visible so you recognise barn and börn as one word the moment you meet them.
Common Mistakes
❌ Treating börn as an unrelated word you must learn separately from barn
Incorrect — börn IS barn, with the stem a rounded to ö by the old plural u. Same noun, predictable alternation.
✅ barn → börn — same word, a rounded to ö
child → children
❌ *dagum — keeping the a in the dative plural
Incorrect — the -um ending contains u, which rounds the stem a: it must be dögum.
✅ dögum — a → ö before the -um ending
(to) days, dative plural
❌ *kallum — leaving the a unrounded in 'we call'
Incorrect — the 1pl ending -um triggers u-umlaut just as in nouns: köllum.
✅ köllum — a → ö before -um
we call
❌ Pronouncing the ö of börn like the o of bord (a back rounded [ɔ])
Incorrect — ö is a FRONT rounded vowel [œ], the same place as a/ɛ but rounded, not a back o-sound.
✅ börn — ö = [œ], front rounded
children
Key Takeaways
- U-umlaut is one rule: a following u rounds a stem a into ö ([a] → [œ]).
- It is the same vowel quality, just rounded — börn is barn with rounded lips, not a separate word.
- The trigger u is sometimes still visible in the ending (dögum, köllum, tölum) and sometimes lost long ago, leaving only ö behind (börn, lönd).
- It fires automatically across the grammar: noun plurals, the dative plural -um, the 1pl verb -um, and adjective agreement.
- Unlike the dead English fossils foot → feet, Icelandic's a → ö is a living, predictable process — learn the trigger, not a word list.
Now practice Icelandic
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Start learning Icelandic→Related Topics
- Strong Feminine Nouns: OverviewA2 — The strong feminine declensions — marked by a genitive singular in -ar (or -ur/-r) and plurals in -ir or -ar — where the singular is almost invariant and all the action is in the plural and its umlaut.
- Weak Verbs: The Four ClassesA2 — The weak verb system — verbs that build their past tense with a dental suffix (-aði, -di, -ði, -ti) instead of a vowel change — split into four classes by their thematic vowel and present pattern, including the Class-4 j-verbs that hide a strong-looking e→a shift inside a weak conjugation.