1. The thematic or etymological classification of phraseological units suggested by l.P.Smith.

On this principle, idioms are classified according to their sorce of origin, things referring to the particular sphere of human activity, of life, of nature, of natural phenomena. L.P.Smith gives in his classification groups of idioms used by sailors, fishermen, soldiers, hunters and associated with the phenomena and conditions of their occupations. This principle of classification is sometimes called etymological. He also makes a special study of idioms borrowed from other languages, but that is only a relatively small part of his classification system.

Here the phraseological units are grouped according to the names of domestic and wild animals, arts, culture, birds, sport, cooking, agriculture, fishermen, sailors.

The thematic principle of classifying phraseological units has real merit but it does not take into consideration the linguistic characteristic features of the phraseological units.

2. The semantic classification of phraseological units suggested by V.V.Vinogradov.

Academician Vinogradov classifies phraseological units which was based on the semantic principle and it is considered by some linguists of today to be outdated, because it was the first classification system. Here phraseological units can be classified according to the degree of motivation of their meaning. He pointed out three types.

1) Fusions where the degree of motivation is very low, we cannot guess the meaning of the whole from the meanings of its components, they are highly idiomatic and cannot be translated word for word into other languages. These are word-groups with a completely changed meaning, but in contrast to the unities they are demotivated, that is, their meaning cannot be deduced from the meanings of the constituent parts.

2) Unities where the meaning of the whole can be guessed from the meanings of its components, but it is transferred (metaphorical or metonymical). These are word-groups with a completely changed meaning, that is, the meaning of the unit does not correspond to the meaning of its constituent parts. They are motivated units or putting it in another way, the meaning of the whole unit can be deduced from the meanings of the constituent parts.

3) Collocations or phraseological combinations where words are combined in their original meaning but their combinations are different in different languages. These are word-groups with a partially changed meaning. They may be said to be clearly motivated, that is, the meaning of the unit can be easily deduced from the meanings of its constituents. This group contains one component used in its direct meaning, while the other is used metaphorically.

3. The structural classification of phraseological units suggested by a.I.Smirnitsky.

He classified two types. He offered a classification system for English phraseoogical units which is interesting as an attempt to combine the structural and the semantic principles. Here phraseological units are grouped according to the number and semantic significance of their constituent parts.

1) One-top or one-summit. Among one-top he pointed out three structural types;

a) units of the type (verb+postposition type) or verbal-adverbial units. They are equivalent to verbs in which the semantic and the grammatical centres coincide in the first constituent

b) units of the type “to be tired” or units equivalent to verbs which have their grammatical centre in the first constituent but their semantic centre in the second. Some of the units remind the Passive voice in their structure but they have different prepositions with them, while in the Passive voice we can have only prepositions “by or with”

c) prepositional-substantive units either to adverbs or to copulas and having their semantic centre in the substantive constituent and no grammatical centre

One-summit units, which have one meaningful constituents.

2) Two-top or two-summit, multi-summit which have two or more meaningful constituents. They are classified into:

a) attributive-substantive or attributive-nominal equivalent to nouns (black art);

b) verbal-substantive or verb-nominal equivalent to verbs (to take the floor);

c) phrasalogical repetitions equivalent to adverbs (now or never, ups and down, back and forth, cakes and ale, as busy as a bee); such units can be built on antonyms, components in repetitions are joined by means of conjunctions. These units are equivalent of adverbs or adjectives and have no grammar centre. They can also be partly or perfectly idiomatic. F: cool as a cucumber (partly), bread and butter (perfectly)

d) adverbial (every other day)