Trope theory, though a minority view today, has been popular at various times throughout the history of philosophy, especially among medieval philosophers. Trope theory holds that properties and relations are themselves (unrepeatable) particulars. (Tropes are also called 'abstract particulars' – 'abstract' in the sense of fine, partial and diffuse, not in the sense of outside space and time.) Thus the redness of a particular billiard ball is a trope, located where the ball is and nowhere else. A different but exactly resembling billiard ball has a numerically different but exactly resembling redness trope. There is no colour property common to, or instantiated in, both balls (similarly all other properties and relations).