as idioms are used as ready-made units with fixed and constant structures.
How to Distinguish Phraseological Units from Free Word-Groups
This is probably the most discussed — and the most controversial — problem in the field of phraseology. The task of distinguishing between free word-groups and phraseological units is further complicated by the existence of a great number of marginal cases, the so-called semi-fixed or semi-free word-groups, also called non-phraseological word-groups which share with phraseological units their structural stability but lack their semantic unity and figurativeness (e. g. to go to school, to go by bus, to commit suicide).
There are two major criteria for distinguishing between phraseological units and free word-groups: semantic and structural.
Compare the following examples:
A.Cambridge don: I'm told they're inviting more American professors to this university. Isn't it rather carrying coals to Newcastle?
(To carry coals to Newcastle means "to take something to a place where it is already plentiful and not needed". Cf. with the R. В Тулу со своим самоваром.)
B.This cargo ship is carrying coal to Liverpool.
The first thing that captures the eye is the semantic difference of the two word-groups consisting of the same essential constituents. In the second sentence the free word-group is carrying coal is used in the direct sense, the word coal standing for real hard, black coal and carry for the plain process of taking something from one place to another. The first context quite obviously has nothing to do either with coal or with transporting it, and the meaning of the whole word-group is