Within the class of compound nouns we distinguish endосentriс and exocentric compounds.

The great variety of compound types brings about a great variety of classifications.

There are two important peculiarities distinguishing compounding in English from compounding in other languages. Firstly, both immediate constituents of an English compound are free forms, i.e. they can be used as independent words with a distinct meaning of their own. The conditions of distribution will be different but the sound pattern the same, except for the stress. The point may be illustrated by a brief list of the most frequently used compounds studied in every elementary course of English: afternoon, anyway, anybody, anything, birthday, day-off, downstairs, everybody, fountain-pen, grown-up, ice-cream, large-scale, looking-glass, mankind, mother-in-law, motherland, nevertheless, notebook, nowhere, post-card, railway, schoolboy, skating-rink, somebody, staircase, Sunday.

The lexicological aspects of the stone wall problem have been mentioned in connection with compound words. Phrasal verbs of the give up type deserve a more detailed study from the phraseological viewpoint. An almost unlimited number of such units may be formed by the use of the simpler, generally monosyllabic verbs combined with elements that have been variously treated as "adverbs", "preposition-like adverbs", "postpositions of adverbial origin", "postpositives” or even "postpositive prefixes”

The so-called stone wall problem concerns the status of the complexes like stone wall, cannon ball or rose garden. Noun premodifiers of other nouns often become so closely fused together with what they modify that it is difficult to say whether the result is a compound or a syntactical free phrase. Even if this difficulty is solved and we agree that these are phrases and not words, the status of the first element remains to be determined. Is it a noun used as an attribute or is it to be treated as an adjective?

Having discussed the difficulties of distinguishing compounds from phrases, we turn to the problem of telling compounds from derivatives. The problem of distinguishing a compound from a derivative is actually equivalent to distinguishing a stem from an affix. In most cases the task is simple enough: the immediate constituents of a compound are free forms, likely to occur in the same phonic character as independent words, whereas a combination containing bound forms as its immediate constituents, is a derivative.

As English compounds consist of free forms, it is difficult to distinguish them from phrases. The combination top dog ‘a person occupying foremost place’, for instance, though formally broken up, is neither more nor less analysable semantically than the combination underdog ‘a person who has the worst of an encounter’, and yet we count the first (top dog) as a phrase and the second (underdog) as a word. How far is this justified? In reality the problem is even more complex than this isolated example suggests. Separating compounds from phrases and also from derivatives is no easy task, and scholars are not agreed upon the question of relevant criteria. The following is a brief review of various solutions and various combinations of criteria that have been offered.

Compound words are words consisting of at least two stems which occur in the language as free forms. In a compound word the immediate constituents obtain integrity and structural cohesion that make them function in a sentence as a separate lexical unit. E. g.: I'd rather read a time-table than nothing at all. The structural cohesion of a compound may depend upon unity of stress, solid or hyphenated spelling, semantic unity, unity of morphological and syntactic functioning, or, more often, upon the combined effect of several of these or similar phonetic, graphic, semantic, morphological or syntactic factors.

Words that are made up of elements derived from two or more different languages are called hybrids. English contains thousands of hybrid words, the vast majority of which show various combinations of morphemes coming from Latin, French and Greek and those of native origin. Thus, readable has an English root and a suffix that is derived from the Latin -abilis and borrowed through French. Moreover, it is not an isolated case, but rather an established pattern that could be represented as English stem+-able. Cf. answerable, eatable, likable, usable. Its variant with the native negative prefix un- is also worthy of note: un-+English stem+-able. The examples for this are: unanswerable, unbearable, unforeseeable, unsayable, unbelievable.

It has already been mentioned in the beginning of this chapter that there exist linguistic forms which in modern languages are used as bound forms although in Greek and Latin from which they are borrowed they functioned as independent words. The question at once arises whether being bound forms, they should be treated like affixes and be referred to the set of derivatives, or whether they are nearer to the elements of compounds, because in languages from which they come they had the status of words. In fact we have a fuzzy set whose elements overlap with the set of affixes on the one hand and with that of words on the other. Different lexicographers have treated them differently but now it is almost universally recognised that they constitute a specific type of linguistic units.