Radial structure, or graded centraility, is a primary feature of a prototype category. It is a center-periphery taxonomy with the abstract or schematic prototype placed at the center. Those instances of category C that display the highest degree of C-ness are placed towards the center, and are more prototypical instances of category C, while those that display a low degree of C-ness are placed towards the periphery, and are less prototypical intances of category C (and are often also extensions of the category).