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A and O as each other's mirror image? Problems with markedness reversal

Abstract This article provides a critical examination of the widespread idea that A and O (and Agents and Patients) are each other's mirror image in terms of case marking and semantic features. We examine patterns of differential case marking for nouns in a sample of 185 languages, showing that the behaviour of A cannot be captured in terms of markedness reversal based on models for O marking. We highlight pecularities of A marking that are not found with O marking, like sensitivity to the feature of motivity, and syntactic restrictions on use as A, and use these to develop an independent account of A marking.