Halliday (2008: 142):
This blending [of the ideational and the interpersonal into a lexicogrammar] entailed a third component, that we call the textual, whereby the meanings are organised into discourse in such a way that each element in this experiential-interpersonal complex (each clause, since that is the key grammatical unit where most of the blending takes place) makes sense with its surroundings, both its material environment of what is going on around and its semiotic environment of other clauses that have gone before.