To know how a sentence is constructed we are to see how its constituents are built and classified. Theoretical syntax describes free word-combinations of the type ( a beautiful girl, ladies and gentlemen, run quickly, on account of etc.). Here the elements are separable, whereas in a phraseological unit, especially in an idiom (to smell a rat ) the globality of nomination reigns supreme over the formal separability of units.
There exist various terms to name a group of words (a word group, a word combination, a cluster of words, a syntagmatic grouping, a phrase, etc.). Russian linguists prefer to use the term “word combination”, Western linguists resort to the term “phrase”. The constituents of a phrase are the head element, the modified one, the governing element; an adjunct, a modifier, the governed element ( in the phrase “a beautiful girl” a girl is the head, while “beautiful” is an adjunct).