Halliday & Matthiessen (2004: 571):
They derive from either the paradigmatic or syntagmatic organisation of lexis.
- The paradigmatic relations are inherent in the organisation of lexis as a resource … . They can be interpreted in terms of elaboration and extension …
- The syntagmatic relations hold between lexical items in a syntagm that tend to occur together, or collocate with one another. …
Since syntagmatic organisation and paradigmatic organisation represent two different dimensions of patterning, any pair of lexical items can involve both.