A central goal in research on visual perception is to understand how the visual system represents the shapes of objects. According to many theorists, axes defined on the basis of object geometry provide a coordinate system for representing the locations and orientations of object parts. An important question that has received little attention concerns how object axes are defined—that is, what aspects of object geometry determine how axes are assigned to shapes? We evaluated 2 hypotheses. According to the elongated-part hypothesis, axes are defined on the basis an object’s most elongated part, such that, for example, the principal axis for a hatchet would coincide with the long axis of the hatchet’s handle. In contrast, the global-shape hypothesis holds that axes are defined on the basis of an object’s overall shape (e.g., for the hatchet, as the longest axis that spans the entire hatchet). Using a novel paradigm involving analysis of mirror-image confusions, we obtained evidence strongly supporting the elongated-part hypothesis. Our results also point to a role for secondary as well as principal axes in object shape representation.

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