Apostrophe s or ss - When to use each? [duplicate] 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 repetitive or awkward combinations of sounds in the