In the present account, grammatical and phonological structures are both trees composed of hierarchies of strings; but it remains the case that semantic structures need by no means have the same shape as structures at any other level. All that the term ʻstructureʼ implies is that there will be some configurations of functions at that stratum, and that these will realize the meaning selections, the combinations of options in the meaning potential.
- Halliday (1973), a pag.95

It is important to emphasize here that structure is defined as the ʻconfiguration of functionsʼ, since this abstract enough to cover semantic structure if such a thing is to be formulated. The shape of a structure may vary; we may express it lineally or hierarchically or simultaneously. But all such shapes have in common the property of being configurations of functions.
- Halliday (1973), a pag.95