I-Umlaut as a Sound Alternation

If u-umlaut is the living vowel alternation of Icelandic — barn → börn, firing automatically every time a u lands in the next syllable — then i-umlaut (i-hljóðvarp) is its fossil cousin. It is an older fronting process: centuries ago, an i or j in a following syllable reached back and pulled the stem vowel forward in the mouth, raising a to e, ó to æ, u to y, and so on. Then that triggering i/j mostly disappeared, leaving only the fronted vowel as a permanent scar. That is why maður has the plural menn, why fótur becomes fætur, why stór yields the comparative stærri. This page makes the alternation visible at the level of sound. The crucial difference from u-umlaut: i-umlaut is no longer productive — it is locked into specific plurals, comparatives, and derived words, so you learn the affected sets rather than applying a live rule. (For the still-active a → ö rounding, see the u-umlaut page.)

What i-umlaut is: fronting, not rounding

U-umlaut rounds (a → ö, lips pushed forward). I-umlaut fronts and often raises — it moves the tongue forward and up toward the position of i, because the trigger was an i. The vowel assimilates toward the i that once followed it. So where u-umlaut pulls a vowel toward [œ], i-umlaut pulls it toward the front [ɛ], [ɪ], [ai] region.

Here is the full set of correspondences. Each row is "what the older back/low vowel becomes when an i/j once followed it":

Notice that two of the outcomes are not plain letters but accented vowels and a diphthong: á and ó both front to æ, and au fronts to ey. So i-umlaut is not only a "change the vowel letter" operation — it can swap an accent (ó → æ) or reshape a whole diphthong (au → ey). This is the same au → ey alternation that the spelling pages exploit to predict when to write ey.

Why it is no longer productive

Here is the key contrast with u-umlaut, and the reason your learning strategy has to be different. U-umlaut is alive: invent a brand-new word with a stem a and tack on a -um ending, and a native speaker will automatically round the a to ö. The rule fires on anything. I-umlaut does not do this. There is no longer any i or j sitting in these endings to trigger it — the trigger eroded away centuries ago — so the alternation is frozen into a fixed list of words and produces nothing new. A modern loanword like gír ("gear") simply pluralises gírar with no umlaut; the process is dead for new vocabulary.

Practically, that means you cannot predict an i-umlaut plural the way you can predict a u-umlaut form. You meet maður/menn, fótur/fætur, bók/bækur as lexical facts to memorise, grouped by the sets below. The payoff is that the sets are small and high-frequency — a few dozen nouns and a handful of comparatives carry almost all of it.

Where i-umlaut lives: three sets

1. Irregular noun plurals. A small set of very common nouns front their stem vowel in the plural instead of just adding an ending. These are exactly the nouns that feel "irregular" until you see the umlaut behind them.

2. Comparatives and superlatives. A group of adjectives front their stem vowel in the comparative/superlative rather than just adding -ari/-astur. This is the i-umlaut showing up in adjective comparison.

3. Derived words. A verb or noun derived from a base often fronts the base's vowel, because the old derivational suffix carried an i/j. This is where i-umlaut quietly links word-families.

The hidden payoff: it links word-families

Because i-umlaut connects a base vowel to its fronted partner, it quietly ties together words that look unrelated. Full ("full") and fylla ("to fill") look like two different words; the umlaut reveals they are one root. Dómur ("judgment") and dæma ("to judge"); sonur ("son") and synir ("sons"); fótur ("foot") and the place-name element fæti-. Once you can see the u ~ y, ó ~ æ, a ~ e correspondences, a single word teaches you a whole cluster. This is also the same fossil exploited on the spelling pages: a y or an æ is usually the umlauted partner of a u/o or an á/ó you can find elsewhere in the family.

How this differs from English

English has the same Germanic i-umlaut — it is the source of man → men, foot → feet, mouse → mice, goose → geese, tooth → teeth. The match with Icelandic is exact and not a coincidence: maður → menn and man → men are the same alternation inherited from the common ancestor. The difference is how much survived. English kept only a handful of frozen plurals and nothing systematic; Icelandic kept the alternation across many nouns, the comparative system, and a productive layer of derivation, so it touches far more of the grammar. For an English speaker that is good news: the concept is already familiar from foot/feet, and Icelandic simply has more of it — but, like English, you memorise the words, because in neither language is i-umlaut a live rule any more.

Common Mistakes

Key Takeaways

• I-umlaut (i-hljóðvarp) is an older fronting alternation: a lost i/j in the next syllable pulled the stem vowel forward — a→e, o→y, u→y, á/ó→æ, ú→ý, au→ey.
• Outcomes include the accented vowel æ (from á/ó) and the diphthong ey (from au) — accents and diphthongs change, not just plain letters.
• Unlike u-umlaut, i-umlaut is no longer productive: it is frozen into a closed set — irregular plurals (maður/menn, fótur/fætur, bók/bækur), comparatives (stór/stærri, ungur/yngri, langur/lengri), and derived words (full/fylla, dómur/dæma).
• You memorise the affected words, you do not apply a live rule — and a new loanword does not umlaut.
• The same alternation links word-families (full ~ fylla, dómur ~ dæma), so one word can teach you a cluster.
• English has the same umlaut in foot/feet, man/men — Icelandic just kept much more of it, but, as in English, the forms are memorised, not predicted.