It would be interesting now to look at phraseological units from a different angle, namely: how are all these treasures of the language approached by the linguistic science? The very miscellaneous nature of these units suggests the first course of action: they must be sorted out and arranged in certain classes which possess identical characteristics.
But which characteristics should be chosen as the main criteria for such a classification system? The structural? The semantic? Those of degree of stability? Of origin?
It should be clear from the previous description that a phraseological unit is a complex phenomenon with a number of important features, which can therefore be approached from different points of view. Hence, there exist a considerable number of different classification systems devised by different scholars and based on different principles.
The traditional and oldest principle for classifying phraseological units is based on their original content and might be alluded to as "thematic" (although the term is not universally accepted). The approach is widely used in numerous English and American guides to idiom, phrase books, etc. On this principle, idioms are classified according to their sources of origin, "source" referring to the particular sphere of human activity, of life of nature, of natural phenomena, etc. So, L. P. Smith gives in his classification groups of idioms used by sailors, fishermen, soldiers, hunters and associated with the realia, phenomena and conditions of their occupations. In Smith's classification we also find groups of idioms associated with domestic and wild animals and birds, agriculture and cooking. There are also numerous idioms drawn from sports, arts, etc.
This principle of classification is sometimes called "etymological". The term does not seem appropriate since we usually mean something different when we speak of the etymology of a word or word-group: whether the word (or word-group) is native or borrowed, and, if the latter, what is the source of borrowing. It is true that Smith makes a special study of idioms borrowed from other languages, but that is only a relatively small part of his classification system. The general principle is not etymological.
Smith points out that word-groups associated with the sea and the life of seamen are especially numerous in English vocabulary. Most of them have long since developed metaphorical meanings which have no longer any association with the sea or sailors. Here are some examples,
To be all at sea — to be unable to understand; to be in a state of ignorance or bewilderment about something (e. g. How can I be a judge in a situation in which I am all at sea? I'm afraid I'm all at sea in this problem). V. H. Collins remarks that the metaphor is that of a boat tossed about, out of control, with its occupants not knowing where they are. 
To sink or swim — to fail or succeed (e. g. It is a case of sink or swim. All depends on his own effort.)
In deep water — in trouble or danger.
In low water, on the rocks — in strained financial circumstances.
To be in the same boat with somebody — to be in a situation in which people share the same difficulties and dangers (e. g. I don't like you much, but seeing that we're in the same boat I'll back you all I can). The metaphor is that of passengers in the life-boat of a sunken ship.
To sail under false colours — to pretend to be what one is not; sometimes, to pose as a friend and, at the same time, have hostile intentions. The metaphor is that of an enemy ship that approaches its intended prey showing at the mast the flag ("colours") of a pretended friendly nation.
To show one's colours — to betray one's real character or intentions. The allusion is, once more, to a ship showing the flag of its country at the mast.
To strike one's colours — to surrender, give in, admit one is beaten. The metaphor refers to a ship's hauling down its flag (sign of surrender).
To weather (to ride out) the storm — to overcome difficulties; to have courageously stood against misfortunes.
To bow to the storm — to give in, to acknowledge one's defeat.
Three sheets in(to) the wind (sl.) — very drunk.
Half seas over (sl.) — drunk.
Though, as has been said, direct associations with seafaring in all these idioms have been severed, distant memories of the sea romance and adventure still linger in some of them. The faint sound of the surf can still be heard in such phrases as to ride out the storm or breakers ahead! (@ Take care! Danger'). Such idioms as to sail under false colours, to nail one's colours to the mast (@ to be true to one's convictions, to fight for them openly) bring to mind the distant past of pirate brigs, sea battles and great discoveries of new lands.
It is true, though, that a foreigner is more apt to be struck by the colourfulness of the direct meaning of an idiom where a native speaker sees only its transferred meaning, the original associations being almost fully forgotten. And yet, when we Russians use or hear the idiom первая ласточка, doesn't a dim image of the little bird flash before our mind, though, of course, we really mean something quite different? When we say на воре и шапка горит, are we entirely free from the picture built up by the direct meanings of the words? If it were really so and all the direct associations of the idioms had been entirely erased, phraseology would not constitute one of the language's main expressive resources. Its expressiveness and wealth of colour largely — if not solely — depend on the ability of an idiom to create two images at once: that of a ship safely coming out of the storm — and that of a man overcoming his troubles and difficulties (to weather/ride out the storm); that of a ship's crew desperately fighting against a pirate brig — and that of a man courageously standing for his views and convictions (to nail one's colours to the mast). .,
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.
The considerable contribution made by Russian scholars in phraseological research cannot be exaggerated. We have already mentioned the great contribution made by Academician V. V. Vinogradov to this branch of linguistic science.
The classification system of phraseological units devised by this prominent scholar is considered by some linguists of today to be outdated, and yet its value is beyond doubt because it was the first classification system which was based on the semantic principle. It goes without saying that semantic characteristics are of immense importance in phraseological units. It is also well known that in modern research they are often sadly ignored. That is why any attempt at studying the semantic aspect of phraseological units should be appreciated.
Vinogradov's classification system is founded on the degree of semantic cohesion between the components of a phraseological unit. Units with a partially transferred meaning show the weakest cohesion between their components. The more distant the meaning of a phraseological unit from the current meaning of its constituent parts, the greater is its degree of semantic cohesion. Accordingly, Vinogradov classifies phraseological units into three classes: phraseological combinations, unities and fusions(R. фразеологические сочетания, единства и сращения). 
Phraseological combinations 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.
E. g. to be at one's wits' end, to be good at something, to be a good hand at something, to have a bite, to come off a poor second, to come to a sticky end (coil.), to look a sight (coil.), to take something for granted, to stick to one's word, to stick at nothing, gospel truth, bosom friends.
Phraseological unities are word-groups with a completely changed meaning, that is, the meaning of the unit does not correspond to the meanings of its constituent parts. They are motivated units or, putting it another way, the meaning of the whole unit can be deduced from the meanings of the constituent parts; the metaphor, on which the shift of meaning is based, is clear and transparent.
E. g. to stick to one's guns (@ to be true to one's views or convictions. The image is that of a gunner or guncrew who do not desert their guns even if a battle seems lost); to sit on the fence (@ in discussion, politics, etc. refrain from committing oneself to either side); to catch/clutch at a straw/straws (@ when in extreme danger, avail oneself of even the slightest chance of rescue); to lose one's head (@ to be at a loss what to do; to be out of one's mind); to lose one's heart to smb. (@ to fall in love); to lock the stable door after the horse is stolen (@ to take precautions too late, when the mischief is done); to look a gift horse in the mouth (@ to examine a present too critically; to find fault with something one gained without effort); to ride the high horse (@ to behave in a superior, haughty, overbearing way. The image is that of a person mounted on a horse so high that he looks down on others); the last drop/straw (the final culminating circumstance that makes a situation unendurable); a big bug/pot, sl.(a person of importance); a fish out of water (a person situated uncomfortably outside his usual or proper environment).
Phraseological fusions 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; the metaphor, on which the shift of meaning was based, has lost its clarity and is obscure.
E. g. to come a cropper (to come to disaster); neck and crop (entirely, altogether, thoroughly, as in: He was thrown out neck and crop. She severed all relations with them neck and crop.); at sixes and sevens (in confusion or in disagreement); to set one's cap at smb. (to try and attract a man; spoken about girls and women. The image, which is now obscure, may have been either that of a child trying to catch a butterfly with his cap or of a girl putting on a pretty cap so as to attract a certain person. In Vanity Fair: "Be careful, Joe, that girl is setting her cap at you."); to leave smb. in the lurch (to abandon a friend when he is in trouble); to show the white feather (to betray one's cowardice. The allusion was originally to cock fighting. A white feather in a cock's plumage denoted a bad fighter); to dance attendance on smb. (to try and please or attract smb.; to show exaggerated attention to smb.).
It is obvious that this classification system does not take into account the structural characteristics of phraseological units. On the other hand, the border-line separating unities from fusions is vagueand even subjective. One and the same phraseological unit may appear motivated to one person (and therefore be labelled as a unity) and demotivated to another (and be regarded as a fusion). The more profound one's command of the language and one's knowledge of its history, the fewer fusions one is likely to discover in it.
The structural principle of classifying phraseological units is based on their ability to perform the same syntactical functions as words. In the traditional structural approach, the following principal groups of phraseological units are distinguishable.
A. Verbal. E. g. to run for one's (dear) life, to get (win) the upper hand, to talk through one's hat, to make a song and dance about something, to sit pretty (Amer. sl.).
B. Substantive. E. g. dog's life, cat-and-dog life, calf love, white lie, tall order, birds of a feather, birds of passage, red tape, brown study.
C. Adjectival. E. g. high and mighty, spick and span, brand new, safe and sound. In this group the so-called comparative word-groups are particularly expressive and sometimes amusing in their unanticipated and capricious associations: (as) cool as a cucumber, (as) nervous as a cat, (as) weak as a kitten, (as) good as gold (usu. spoken about children), (as) pretty as a picture, as large as life, (as) slippery as an eel, (as) thick as thieves, (as) drunk as an owl (sl.), (as) mad as a hatter/a hare in March.
D. Adverbial. E. g. high and low (as in They searched for him high and low), by hook or by crook (as in She decided that, by hook or by crook, she must marry him), for love or money (as in He came to the conclusion that a really good job couldn't be found for love or money), in cold blood (as in The crime was said to have been committed in cold blood), in the dead of night, between the devil and the deep sea (in a situation in which danger threatens whatever course of action one takes), to the bitter end (as in to fight to the bitter end), by a long chalk (as in It is not the same thing, by a long chalk).
E. Interjectional. E. g. my God! by Jove! by George! goodness gracious! good Heavens! sakes alive! (Amer.)
Professor Smirnitsky offered a classification system for English phraseological units which is interesting as an attempt to combine the structural and the semantic principles  Phraseological units in this classification system are grouped according to the number and semantic significance of their constituent parts. Accordingly two large groups are established:
A. one-summit units, which have one meaningful constituent (e. g. to give up, to make out, to pull out, to be tired, to be surprised1);
B. two-summit and multi-summit units which have two or more meaningful constituents (e. g. black art, first night, common sense, to fish in troubled waters).
Within each of these large groups the phraseological units are classified according to the category of parts of speech of the summit constituent. So, one-summit units are subdivided into: a) verbal-adverbial units equivalent to verbs in which the semantic and the grammatical centres coincide in the first constituent (e. g. to give up); b) units equivalent to verbs which have their semantic centre in the second constituent and their grammatical centre in the first (e. g. to be tired); c) prepositional-substantive units equivalent either to adverbs or to copulas and having their semantic centre in the substantive constituent and no grammatical centre (e. g. by heart, by means of).
Two-summit and multi-summit phraseological units are classified into: a) attributive-substantive two-summit units equivalent to nouns (e. g. black art),
b) verbal-substantive two-summit units equivalent to verbs (e. g. to take the floor), c) phraseological repetitions equivalent to adverbs (e. g. now or never);
d) adverbial multi-summit units (e. g. every other day).
Professor Smirnitsky also distinguishes proper phraseological units which, in his classification system, are units with non-figurative meanings, and idioms, that is, units with transferred meanings based on a metaphor.
Professor Koonin, the leading Russian authority on English phraseology, pointed out certain inconsistencies in this classification system. First of all, the subdivision into phraseological units (as non-idiomatic units) and idioms contradicts the leading criterion of a phraseological unit suggested by Professor Smirnitsky: it should be idiomatic.
Professor Koonin also objects to the inclusion of such word-groups as black art, best man, first night in phraseology (in Professor Smirnitsky's classification system, the two-summit phraseological units) as all these word-groups are not characterized by a transferred meaning. It is also pointed out that verbs with post-positions(e. g. give up) are included in the classification but their status as phraseological units is not supported by any convincing argument.
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The classification system of phraseological units suggested by Professor A. V. Koonin is the latest outstanding achievement in the Russian theory of phraseology. The classification is based on the combined structural-semantic principle and it also considers the quotient of stability of phraseological units.
Phraseological units are subdivided into the following four classes according to their function in communication determined by their structural-semantic characteristics.
1. Nominative phraseological units are represented by word-groups, including the ones with one meaningful word, and coordinative phrases of the type wear and tear, well and good.
The first class also includes word-groups with a predicative structure, such as as the crow flies, and, also, predicative phrases of the type see how the land lies, ships that pass in the night.
2. Nominative-communicative phraseological units include word-groups of the type to break the ice — the ice is broken, that is, verbal word-groups which are transformed into a sentence when the verb is used in the Passive Voice.
3. Phraseological units which are neither nominative nor communicative include interjectional word-groups.
4. Communicative phraseological units are represented by proverbs and sayings.
These four classes are divided into sub-groups according to the type of structure of the phraseological unit. The sub-groups include further rubrics representing types of structural-semantic meanings according to the kind of relations between the constituents and to either full or partial transference of meaning.
The classification system includes a considerable number of subtypes and gradations and objectively reflects the wealth of types of phraseological units existing in the language. It is based on truly scientific and modern criteria and represents an earnest attempt to take into account all the relevant aspects of phraseological units and combine them within the borders of one classification system.