It would not be quite correct to assert that predicative words are exceptional in English. In keeping with the theory of nominalisation some basic predicative structures are apt to the transformation of nominalisation. The result of nominalisation is the derivation of noun-phrases. Accordingly, predicative relations characteristic of predicative constructions become concealed by the overt attributive -relations between the constituents of a noun-phrase: 1) N is A.
T = NisA—>A + N
The sea is stormy — + the stormy sea
2) N is N,—>Ni + N
Davidson is a doctor — * doctor Davidson
In both cases the resultant noun-phrases assume the form of attributive word-groups though the predicative character of the relations inherent in their constituents is re-establishable due to the transformation suggested above.
^ Nominalised word-groups, in their turn, are apt to lexicalisation the result of which is a phrase-word or a compound word. That is why it is convenient to comment upon.the implication and concealment of predicativity both in nominalised word-groups and in compound nouns. The process of lexicalisation is gradual, with many intermediate cases illustrating the graduality of implication of • predicative relations. It is sometimes difficult to qualify a combination of elements as either a word-group, a phraseological unit, or a compound word.
Compare: a night duty, home work, population growth, heat pipe, space ship, machine time, errand-boy, etc.