Groups Of Words Of The Same Primary Class

Matthiessen (1995: 627):

To sum up, groups differ from phrases in that 

(i) they are groups of words of the same primary class (nominals, verbs, or adverbials), so their functional potential is related to the Head word;

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To be clear, there are exceptions to this generalisation, as when a verb serves as the Epithet or Classifier of a nominal group, or an adverb serves as Sub-Modifier; see Halliday & Matthiessen (2014: 427).