One of the many extraordinary features of Han'gŭl is its uniformity and systematic purity. Each Han'gŭl sign, that is, each unit of interpretation, relates to the same kind of linguistic unit, a syllable, in the same manner by building it up from smaller components, which in turn relate to parts and aspects of the articulated sound stream in a uniform manner. This makes Han'gŭl one of the most systematically coherent scripts ever invented and used. Most other writing systems are much less consistent, incorporating as they do a variety of units and relying on a variety of mapping relations for interpretation. The reasons for this lie in the principle of historicity, which, as pointed out in chapter 1, is common to all writing systems. From their inception to their mature form all writing systems have gone through an extended process of evolution, often retaining features of earlier stages. New features were added, others discarded, as the system evolved alongside, but not necessarily in close connection with, the language it was first used to write. As a consequence, the mapping relation with the language underwent changes, too, often getting more complicated.