Halliday & Matthiessen (2014: 26):
Language is thus organised into four strata – semantics, lexicogrammar, phonology, and phonetics. But these four strata are grouped into two stratal planes, the content plane and the expression plane. When children learn how to mean, they start with a very simple semiotic system, a protolanguage, usually sometime in the second half of their first year of life (see Halliday, 1973, 2003); and we hypothesise that language evolved in the same way (see Matthiessen, 2004a). This system is organised into two stratal planes, content and expression; but neither is internally stratified: content is mapped directly onto expression (vocal or gestural).