Metaphor and metonymy do not occur in isolation; they are triggered in utterances in particular linguistic (and extralinguistic) contexts. They pose an interesting problem from the point of view of semantic composition in that the metaphorical or metonymic interpretation of the parts (the individual words) appears to be determined by the interpretation of the whole construction in which they are found. Much of this is determined by the domain in which the words are to be interpreted. Domains play a central role in the definition of a metaphor as a mapping of conceptual structure from one domain to another. Domains also play a significant (though not defining) role in most metonymies and some related lexical ambiguities, as the highlighting of particular domains in a domain matrix. The processes of domain mapping and domain highlighting are governed by the requirement that a dependent predication (in the sense of Langacker 1987) and all of the autonomous predications it is dependent on must be interpreted in a single domain; this is "the conceptual unity of domain." This is only one of several "conceptual unities" imposed by a whole construction on its component parts. (William Croft)