In this experiment, we investigate the effect of differences in the shape of category extensions on learning when the volumes of the to-be-learned categories do not vary. Recall that the shape or compactness of the category concerns the number of dimensions (or directions in the space of all possible objects) on which there is a restricted range of values within the category. In order to determine how important compactness, independent of volume, is for learning by a simple associative device, we contrasted compact noun-like categories with less compact adjective-like categories of the same volume.