-'s

-'s¹ [s, z after a vowel sound or voiced consonant, ɪz after a sibilant]

suffix denoting possession in singular nouns, also in plural nouns not having a final -s: John's book.

Origin

OE, masculine and neut. genitive singular ending.

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-'s² [s, z after a vowel sound or voiced consonant, ɪz after a sibilant]

suffix denoting the plural of a letter or symbol: 9's.