This paper aims at demonstrating that the cognitive mechanisms underlying certain tropes (e.g. metaphor or metonymy) may assume variable degrees of conventionalisation, thereby giving rise to a range of phenomena along either side of the semantics/ pragmatics distinction. Examining specifically cases of metonymy, I propose a pragmatic account of creative, one-off metonymic expressions using the framework of relevance theory; my main argument is that metonymy is a variety of the interpretive use of language. I further look at degrees of conventionalisation that a given metonymy may go through until it becomes fully semanticised, thus bringing about semantic change. My discussion should have farreaching implications for lexical semantics and the relevance-theoretic distinction between descriptive and interpretive use.