Halliday (2008: 186):
The third complementarity is the outcome of the child’s learning to read and write; it means gaining access to the new dimension that was added to language by the evolution of writing systems. […] writing opened the way to a new and different mode of meaning, that we refer to as “written language”; that is where the complementarity resides, and full mastery of it not only takes a long time but also depends on maturation: it is simply not possible (or has not been up until now) for an eight- or nine-year-old to cope with the type of semiotic complexity that written language involves.