1. On the use of so-called 'zero genitive', marked by a simple apostrophe in spelling ('), as opposed to the 's genitive, Quirk, Greenbaum, Leech and Svartvik specify in A Comprehensive grammar of the English Language (pp. 320 & 321) that: In addition to its normal use with regular plurals such as boys', the 'zero genitive' is used to avoid ... More @Wikipedia
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