Summary

Jakob Jakobsen An Etymological Dictionary of the Norn Language in Shetland…

The phonology of the Norn words handed down is on the whole somewhat confused, largely owing to the strong Scottish influence, an influence which has made the vowel-system very diversified. In a large number of cases, different forms of a word exist side by side (differing in different districts, but sometimes differing in one and the same district) , frequently representing different stages of development.
Source: Wikisource

Jakob Jakobsen An Etymological Dictionary of the Norn Language in Shetland…

As the i-mutation is so old and so usual a phenomenon in Norse as well as in German, it is almost as a matter of course that it appears in Shetland Norn. There is, however, the peculiarity that the i-mutation is wanting in a great many words in Shetland Norn, in words too, where no closely related words with an original vowel are to be found that could have a smoothing effect and abolish the mutation.
Source: Wikisource

Jakob Jakobsen An Etymological Dictionary of the Norn Language in Shetland…

Whether any u-mutation appears in the Shetlandic word cannot be proved by the vowel-sound.
The u-mutation can be exactly proved in *gøda [gøda, ꬶøda] , sb., way, path, as ø here can only be developed from ǫ in “gǫtu”, nom.: gata. Besides *gøda, the form *goda [gȯda, ꬶȯda] , and (rarely) *gota [ꬶȯta] , is found, and prob. on account of the [ȯ] -sound, must be accepted as a mutated form. “goda, gøda”, now only a place-name, is remembered as a common noun, esp. in Fe. gødins [gødɩns] , in “de Midgødins” (Y.) , is def. pl. form: O.N. gǫturnar, with a later added Eng. pl. s.
Source: Wikisource

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