Here, though, we are on dangerous ground because the border-line dividing phraseological units with partially changed meanings from the so-called semi-fixed or non-phraseological word-groups (marginal cases) is uncertain and confusing.
The term "idiom", both in this country and abroad, is mostly applied to phraseological units with completely transferred meanings, that is, to the ones in which the meaning of the whole unit does not correspond to the current meanings of the components. There are many scholars who regard idioms as the essence of phraseology and the major focus of interest in phraseology research.
The structural criterion also brings forth pronounced distinctive features characterising phraseological units and contrasting them to free word-groups.
Structural invariability is an essential feature of phraseological units, though, as we shall see, some of them possess it to a lesser degree than others. Structural invariability of phraseological units finds expression in a number of restrictions.
First of all, restriction in substitution. As a rule, no word can be substituted for any meaningful component of a phraseological unit without destroying its sense. To carry coals to Manchester makes as little sense as Б Харьков со своим самоваром.
The idiom to give somebody the cold shoulder means "to treat somebody coldly, to ignore or cut him", but a warm shoulder or a cold elbow make no sense at all. The meaning of a bee in smb's bonnet was explained above, but a bee in his hat or cap would sound a silly error in choice of words, one of those absurd slips that people are apt to make when speaking a foreign language.
At the same time, in free word-groups substitution does not present any dangers and does not lead to any serious consequences. In The cargo ship is carrying coal to Liverpool all the components can be changed: