Past research approached the origins of the Coptic alphabet sociolinguistically and empirically. Neither can fully explain the comparatively sudden and fundamental change from a supraphonemic to a phonemic writing system for Egyptian around the second century AD. This paper adds the cognitive-linguistic concept of the grain size of a writing system to the picture. In essence, by the second century, sound changes in Egyptian had resulted in a phonological structure of the language that mapped more easily onto a phonemic writing system than previous stages of the language. This coincided with socio-political developments favouring the Greek alphabet. As a result, multiple writing systems, which shared the underlying structure, alphabetic, and model, the Greek alphabet, emerged. Eventually, one of these prevailed, the Coptic alphabet.