Halliday & Matthiessen (1999: 278, 279, 280):
Categories in the ideation base are realised by those categories in the ideational grammar with which they have co-evolved. These are the congruent realisations that developed first in the language, are learnt first by children and tend to occur first in a text For instance, a sequence is realised by a clause complex, and the figures related in the sequence are realised by the clauses strung together in the clause complex: see Figure 6-5. …
… The system then comes to be expanded through shift in rank and in class. Sequences come to be realised not only by clause complexes but also by clauses, and figures come to be realised not only by clauses but also by groups/phrases, as shown in Figure 6-6. They are pushed downwards in complexity and rank relative to their congruent realisations in the grammar.
