The Weser Old Frisian a -stem dative plural ending - on is unusual, since the other Old Frisian dialects reflect -um . This article discusses whether -on is the result of affix borrowing from Old Saxon or the result of a regular sound change, guided by typological and dialectological considerations. The truth appears to lie somewhere in the middle: The ending -on is likely the result of a small-scale sound change that only affected the dialects ancestral to Weser Old Frisian, Old Saxon, and Old Dutch.
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Simon den Hertog (2024) studied this question.
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