identical constituents. The difference between a blackbird and a black bird is best explained by their relationship with the grammatical system of the language. The word blackbird, which is characterised by unity, possesses a single grammatical framing: blackbird/s. The first constituent black is not subject to any grammatical changes. In the word-group a black bird each constituent can acquire grammatical forms of its own: the blackest birds I've ever seen. Other words can be inserted between the components which is impossible so far as the word is concerned as it would violate its unity: a black night bird.
The same example may be used to illustrate what we mean by semantic unity.
In the word-group a black bird each of the meaningful words conveys a separate concept: bird — a kind of living creature; black — a colour.
The word blackbird conveys only one concept: the type of bird. This is one of the main features of any word: it always conveys one concept, no matter how many component morphemes it may have in its external structure.
A further structural feature of the word is its susceptibility to grammatical employment. In speech most words can be used in different grammatical forms in which their interrelations are realised.
So far we have only underlined the word's major peculiarities, but this suffices to convey the general idea of the difficulties and questions faced by the scholar attempting to give a detailed definition of the word. The difficulty does not merely consist in the considerable number of aspects that are to be taken into account, but, also, in the essential unanswered questions of word theory which concern the nature of its meaning (see Ch. 7).
All that we have said about the word can be summed up as follows.