NOUNS
are divided into masculine, feminine and neutral nouns. However, not all masculine nouns are inflected the same way; each "stem" has its own inflection. The stem names may seem arbitrary, since there aren't /a/:s in all a-stems, nor are there /n/:s in all n-stems etc., but the division is based on how the words looked in an earlier phase, in which all a-stems had an /a/ before the ending.
The stem of a word is the part of it that the ending is attached to. The stem of konungr thus is konung-, to which endings like -r, -s and -um are attached. An a-stem is a word that long ago had a stem ending with an /a/, like *konungaR and *stainaR. These /a/:s are long gone leaving the forms konungr and steinn (from an older form stæinR through assimilation by /nR/ into /nn/).
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· a-stems · · ia-stems · · ô-stems · · iô-stems · · i-stems · · u-stems · · an-stems · · ôn-stems · · în-stems · · nd-stems · · root nouns · · r-stems ·

masc. and neut. a-, ja- and va-stems · masc. and neut. ia-stems · fem. ô-, jô- and vô-stems · fem. iô-stems · masc. and fem. i-stems · masc. u-stems · masc. and neut. an-stems · fem. ôn-stems · fem. în-stems · masc. nd-stems · masc. and fem. root nouns · masc. and fem. r-stems ·