Stratification: From Protolanguage To Language

Halliday & Matthiessen (2014: 26-7):

This protolanguage is a child tongue rather than a mother tongue; it is not yet like the adult language spoken around young children. Children develop their protolanguages in interaction with their immediate caregivers, gradually expanding their protolinguistic meaning potentials. In doing so, they learn the principles of meaning. At some point, typically in the second year of life, they are ready to build on this experience and to begin to make the transition into the mother tongue spoken around them. This transition involves a number of fundamental changes in the linguistic system. A key change – one that makes possible other changes – is the splitting up of each of the two stratal planes into two content strata and two expression strata. Content gradually splits into semantics and lexicogrammar, and expression gradually splits into phonology and phonetics.