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; the other essential feature of phraseological units is stability of the lexical components and grammatical structure.
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. E.g., in a red flower (a free phrase) the adjective red may be substituted by another adjective denoting colour, and the word-group will retain the meaning: "the flower of a certain colour".
In the phraseological unit red tape (áþðîêðàòè÷åñêèå ìåòîäû) no such substitution is possible, as a change of the adjective would cause a complete change in the meaning of the group: it would then mean "ÿ 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 any change of its lexical components and its grammatical structure.
Grammatical structure of phraseological units is to a certain degree also stable:
red tape - a phraseological unit;
red tapes - a free word-group;
to go to bed - a phraseological unit;
to go to the bed - a free word-group.
Still the basic criterion is comparative lack of motivation, or idiomaticity of the phraseological units. Semantic motivation is based on the coexistence of direct and figurative meaning.