Do symmetrical letter pairs affect readability?

Our everyday experience shows that we have problems in recognizing objects which only differ in their symmetry properties (street signs with two arrows in different directions or mathematical signs such as 〈 and 〉). Perception is closely correlated with an inner comparison: the perceived object with its surrounding, the perceived object with former experience and so on. The brain has evolved different constancy abilities (e.g. colour constancy) and one of them is object constancy. This object constancy makes it possible to perceive an object regardless of its orientation in space. Symmetric letter pairs with different sound representations (such as 〈b〉 and 〈d〉) are, due to object constancy, typically identified as one object. This deficiency of distinctiveness should affect their readability. The above hypothesis was examined in many scripts. The result was that mature scripts (which usually developed for a long time) avoid these symmetric letter pairs (called extrinsic symmetry) by adding distinctive features such as serifs or different stroke thickness. On the other hand, if a writer is allowed to invent letter shapes freely, he makes use of extrinsic symmetrical letter pairs. This is supposed to have aesthetic reasons — letters are often perceived as a standing object or even as a “body” on a plane. It is therefore possible to statistically separate mature scripts which show up no extrinsic symmetry from invented scripts full of extrinsic symmetry. The runes are a writing system which does not quite fit in this widely proved distinction. They have developed from the Latin writing system (or a close relative of it) and have therefore inherited the avoidance of extrinsic symmetry. The reduction of the character set from 24 signs in the Old Futhark to 16 characters in the Younger Futhark is accompanied by a simplification of runic signs. During this period the runes develop a high degree of extrinsic symmetry. Moreover, the letter shapes are often related to different sound representations. These irregularities in usage may be caused by interference from the Latin writing system. The resulting lesser readability could have been one reason for the decline of the runes. This paper shows in many figures and graphs how symmetry emerges and under what circumstances it is used to create new letter shapes.