The U-shift (also known as U-umlaut) is a phonological process (sound change) that affects all classes of words in Icelandic. The basic rule is as follows, where C stands for a consonant:
|a||→||ö (stressed position)|
u (unstressed position)
In other words, a changes to ö if it’s in stressed position (i.e. the first syllable of a word) or u in unstressed position, when followed by u.
There will be one or more consonants in between the a and the u that causes the U-shift. However, if there are any intervening vowels, then the U-shift is blocked and the a remains. For example:
You might wonder why the -ur ending in adjectives and nouns such as glaður and dagur doesn’t cause U-shift. This is because the u here is a relatively recent development (in Old Norse these words were glaðr and dagr respectively), but the U-shift is an older process that’s no longer productive in modern Icelandic.
When words ending in -ur get other endings that start with u, then U-shift does occur (e.g. dative plural glöðum and dögum).
Where’s the u gone?
Sometimes, the u that causes the U-shift to take place is no longer present. This is why U-shift occurs:
- in nominative feminine singular and nominative/accusative neuter plural forms of strong adjectives (e.g. glaður → glöð, dapur → döpur);
- in the plural of strong neuter nouns (e.g. barn → börn, land → lönd);
- in the singular of strong feminine nouns, where the U-shift is then reversed in other forms (e.g. gjöf → gjafir, röð → raðir).
Originally, all of these word forms would have got an ending starting with u that has since been lost, leaving behind only the U-shifted stem vowel.
In some words that have two vowel sounds in the stem, a may fall in the second syllable. When this a is U-shifted, it changes to ö instead of the expected u. For example:
This tends to be the case in loanwords and compound nouns, whereas in words of Icelandic origin, the regular stress rule is followed and a changes to u unless it’s in the first syllable:
There are some exceptions amongst native Icelandic words, however, such as hérað “district, county”, which is commonly héröð in the plural (but héruð also occurs), and líkan “model” (pl. líkön).
Although the U-shift is no longer a productive process, it is applied to new words in Icelandic by analogy with existing ones. See these neuter nouns for example:
This also extends to foreign names, to sometimes amusing effect:
Ertu búin að sjá nýju myndina með Söndru Bullock?