This chapter explores various aspects and examples of analogy and extension, with particular attention to their interrelations. Analogy can be understood as an automatic cognitive process by which what is known is extrapolated to what is considered similar, which leads to similar outcomes, that is, extension. Later cognitive development usually prevents incorrect analogy. Extension following perceptual analogy in conventional semiotic systems produces changes in the system; instead of being interpreted as erroneous, it should be considered as a reflection of evolving human perception. Thus, semiotic systems generally evolve via extension generated by analogy, which can be illustrated by language and graphemics. Analogy between graphemic systems in contact may induce intergraphemic extension, and analogy between different categories of a given graphemic system may lead to intragraphemic extension. Because graphemic systems are related to language in many ways (for example, to elements of phonemic and morphemic systems), analogy concerning graphemics may produce intersemiotic extension, either from language to graphemics or in the opposite direction. The seemingly ideal synchronic correlation between graphemics and linguistic elements may have been caused by historical analogy and extension between them, and this evolution can be studied by diachronic analysis of language–writing relations.