Writing systems avoid characters that are mirror images of one another (like b and d). The reasons for this are bound up with the way our brains work. The present study examines 41 sets of characters and signs in diachronic and contrastive terms. The results of the examination indicate that long-established, functional writing systems are subject to tensions between aesthetically motivated symmetry and the reduction of such symmetry in the interests of improved readability. Invented writing systems where readability is not a criterion (e.g. secret codes) normally draw upon symmetrical characters to extend the number of signs at their disposal.