Word groups may be classified according to different principles: according to the order and arrangement of the component members; according to the criteria of distribution; according to their headword; according to their syntactic pattern; according to the degree of motivation and the degree of the semantic and structural cohesion of the members (motivated, free, or variable word-groups; semi-motivated and non-motivated). Non-motivated word-groups are called stable word- groups, set-phrases, idioms, or phraseological units. They are studied in Phraseology.
