Set expressions are not created in speech, they are introduced into speech ready-made like words. Free word groups are constructed in the process of speaking by joining words together. Set expressions like words possess semantic unity and integrity.
Free word combinations are clearly motivated and easily understood, but the meaning of phraseological units is not the combined meanings of their constituents. Set expressions possess a greater structural unity than free word combinations, though the degree of structural invariability may vary in different set expressions. The structural invariability of set expressions manifests itself in a number of restrictions: the restriction of substitution, the restriction in introducing any additional components into the structure of a phraseological unit, grammatical invariability.
