Of the cases belonging to the Indo-European language, Greek has lost the free use of three: instrumental, locative, and ablative. A few of the forms of these cases have been preserved ( cross341, cross1449, cross1535); the syntactical functions of the instrumental and locative were taken over by the dative; those of the ablative by the genitive. The genitive and dative cases are therefore composite or mixed cases.
N.—The reasons that led to the formation of composite cases are either (1) formal or (2) functional. Thus (1) χώρᾳ is both dat. and loc.; λόγοις represents the instr. λόγοις and the loc. λόγοισι; in consonantal stems both ablative and genitive ended in -ος; (2)
Herbert Weir Smyth [n.d.], A Greek Grammar for Colleges; Machine readable text [info] [word count] [Smyth].