Classic examples of ostensive geometrical constructions are used to clarify Kant’s account of how they provide knowledge of claims about rigid bodies we can observe and manipulate. It is argued that on Kant’s account claims warranted by ostensive constructions must be limited to scales and tolerances corresponding to our perceptual competencies. This limitation opens the way to view Riemann’s work as contributing valuable conceptual resources for extending geometrical knowledge beyond the bounds of observation. It is argued that neither Reichenbach’s descriptions of non-Euclidean visualization nor his arguments for conventionalism about geometry undercut this view of Kant’s account of geometrical knowledge.