R. S. Ginzburg, S. S. Khidekel, G. Y. Knyazeva, A. A. Sankin a course in modern english

§ 12. Criteria of Stability
and Lack of Motivation
(Idiomaticity)

Phraseological units are habitually defined as non-motivated word-groups that cannot be freely made up in speech but are reproduced as ready-made units. This definition proceeds from the assumption that the essential features of
¹ Cf., e. g., the interpretation of these terms in the textbooks on lexicology by I. V. Arnold, A. I. Smirnitsky and in A. V. Kunin’s Англо-русский фразеологический словарь. М., 1967.
² For a different interpretation of the term idiom see: А. И. Смирницкий. Лексикология английского языка. М., 1956,
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phraseological units are stability of the lexical components and lack of motivation.¹ It is consequently assumed that unlike components of free word-groups which may vary according to the needs of communication, member-words of phraseological units are always reproduced as single unchangeable collocations.
Thus, for example, the constituent red in the free word-group red flower may, if necessary, be substituted for by any other adjective denoting colour (blue, white, etc.), without essentially changing the denotational meaning of the word-group under discussion (a flower of a certain colour). In the phraseological unit red tape (bureaucratic “methods) no such substitution is possible, as a change of the adjective would involve a complete change in the meaning of the whole group. A blue (black, white, etc.) tape would mean ‘a tape of a certain colour’. It follows that the phraseological unit red tape is semantically non-motivated, i.e. its meaning cannot be deduced from the meaning of its components and that it exists as a ready-made linguistic unit which does not allow of any variability of its lexical components.
It is also argued that non-variability of the phraseological unit is not confined to its lexical components. Grammatical structure of phraseological units is to a certain extent also stable. Thus, though the structural formula of the word-groups red flower and red tape is identical (A + +N), the noun flower may be used in the plural (red flowers), whereas no such change is possible in the phraseological unit red tape; red tapes would then denote ‘tapes of red colour’ but not ‘bureaucratic methods’. This is also true of other types of phraseological units, e.g. what will Mrs. Grundy say?, where the verbal component is invariably reproduced in the same grammatical form.