Leveling of the radical vowel in the plural paradigm of the consonantal r-stems denoting family relations. In the plural paradigm of the substantive consonantal declension, i-umlaut of the radical vowel originally occurred only in the nom.-acc. form, since only here did the end syllable contain the vowel i. This vowel status is regularly preserved in the monosyllabic consonantal stems; cf. fϕtr (*fōt-iR) fóta fótum fϕtr (*fōt-iR), from fótr (masc.) ‘foot’; merkr (*mark-iR) marka mǫrkum merkr (*mark-iR), from mǫrk (fem.) ‘mark’ (a weight or money value). The consonantal r-stems denoting family relations belonged to the dissyllabic pattern in the singular (faÐer ‘father’, bróÐer ‘brother’, móÐer ‘mother’, dótter ‘daughter’, syster ‘sister‘), but became monosyllabic in the plural (feÐr, brϕÐr, etc.). Nevertheless, contrary to the pattern of the monosyllabic type (fótr, mǫrk), the original radical vowels a and ó in the genitive and dative forms were leveled out in favor of the umlauted vowels e and ϕ of the nom.-acc. forms; cf.
, etc. So far as I know, no explanation has ever been given for this discrepancy between the two types of consonantal stems.