I provide a formal account of the diachronic development of the English possessive marker, from unambiguous affix in Old English, to clitic (in at least some contexts) in Present-Day English. I show that Lexical Sharing, appropriately constrained, makes it possible to represent the degrees of degrammaticalization between full clitic and full affix shown by the English possessive at different periods, while maintaining an absolute distinction between word and morpheme, as is necessary in a lexicalist theory.