As explained by Whitehurst and Lonigan, Clay's concept of emergent literacy stands as an alternative understanding to the traditional idea that literacy beings in a specific moment in time, that being when formal instruction is begun. This is the present commonly used "reading readiness" approach, which draws a distinction between the phases of "prereading" and "reading." The reading readiness approach contends reding acquisition is dependent upon--and thus measures for skill acquisition--categories such as language use and reading concepts.
In contrast to reading readiness and similar approaches, Clay's emergent literacy is dependent on the idea that reading acquisition occurs across a continuum. The emergent reading approach involves the same skill acquisition categories as reading readiness does but includes an additional set of categories of skills. The first categories, shared by reading readiness and emergent literacy approaches, are termed in emergent literacy theory "outside-in skills," which are, as has been said, skills such as language use and reading concepts. The second set of skills, which are particular to emergent literacy, are termed "inside-out skills" and comprise such skills as phonological awareness (language sound differentiation) and letter knowledge.
Emergent literacy theory holds that these two sets of skills come from different experiences and are influential for the child's literacy at different segments of time during reading acquisition. Therefore, in emergent literacy, literacy acquisition originates early in the child's life instead of beginning with formal instruction. Further, because in the emergent literacy approach two skill sets, or "domains," are integral to reading acquisition, there is no demarcated division between prereading and "real" reading.
["Child Development and Emergent Literacy," Grover J. Whitehurst and Christopher J. Longin. Child Development, June 1998, Volume 3, Page 848-872.]