The vocabulary of a language consists not only of words but also word combinations which are differently called. Some of them refer to free word combinations, the others are called set expressions and the term phraseological units is often used for them.
Set expressions are contrasted to semifixed combinations and free phrases. All these are but different stages of restrictions imposed upon co-occurrence of words.
Free phrases or free word combinations are created according to the grammatical rules of the language, in accordance with a certain speech situation with the necessity of expressing a certain idea on the basis of a specific syntactical pattern peculiar for the language. Every notional word functions in free word combinations as a certain member of the sentence. The general sense of such an ordinary free combination is derived from the conjoined meanings of its elements. Thus, for example, a black coat, remarkably intelligent and so on are non-predicative elements of speech, they are separate concepts. They are free in the sense that "black" is not necessarily connected with "coat". We may also say "green coat", "short coat", "great coat", etc. or "black suit", "black shirt", "grey skirt" and so on
Set expressions may be defined as complex word-equivalents which are not produced in speech but reproduced and the globality of nomination reigns supreme over the formal separability of their elements. There is no choice, no freedom in the use of the components of the combinations. They are ready-made units and may be described as idiomatic. Alongside with semantic unity there is the unity of syntactic function. It means that in the sentence they function as a single syntactic unit irrespective of the number of elements. For example: The woman looked at the man to see whether he was pulling her leg" - "Женщина посмотрела на молодого человека: не вздумал ли он