Halliday & Matthiessen (2014: 9-10):
Let us summarise here the five principles of constituency in lexicogrammar.(1) There is a scale of rank in the grammar of every language. That of English (which is typical of many) can be represented as:clause(2) Each consists of one or more units of the rank next below. For example, Come! is a clause consisting of one group consisting of one word consisting of one morpheme.
(3) Units of every rank may form complexes: not only clause complexes but also phrase complexes, group complexes, word complexes and even morpheme complexes may be generated by the same grammatical resources.
(4) There is the potential for rank shift, whereby a unit of one rank may be downranked (downgraded) to function in the structure of a unit of its own rank or of a rank below. Most commonly, though not uniquely, a clause may be down-ranked to function in the structure of a group.
(5) Under certain circumstances it is possible for one unit to be enclosed within another; not as a constituent of it, but simply in such a way as to split the other one into two discrete parts.