(Writing Systems: An introduction to their linguistic analysis, Florian Coulmas)
These technical developments have repercussions on the structure of the signs and the way they are processed. Recognition of the signs is no longer based on similarity but on discrimination, as a pictorial likeness is gradually replaced by the necessity to distinguish one sign from another. Differentiation thus becomes the principal design feature of the signs. For example, that sign of a bull resembles a bull is now less important than that it differs from the sign of a cow.
The relationship between signs and objects is superseded by multiple relationships between signs and other signs as the scribes' chief concern. The signs thus become part of a graphic system characterized by negative differentiation. The underlying principle is that the many signs are to be kept from becoming confused with one another, much like the units of a language. The creation of new signs follows the same principle when lines are added to existing signs or one sign is adjoined to another. Contrast with all other signs become a defining feature of every sign.