Sinclair’s grammatical work is notable for its strict reliance on performance data and its avoidance of psychological theorising. His argument rests on the observation that large corpora reveal a huge discrepancy between the predictions made by cognitive models of grammar and what actually happens in performance. This discrepancy cannot be explained away by appeal to encoding/processing deficiencies, but must be taken as reason to revise our view of grammar, especially by exploring its interdependence with lexis. In this he follows Halliday’s (1966) idea that every word has its own grammar. Sinclair’s 1991 paper on the word of (‘The meeting of lexis and grammar’) exemplifies his thinking in this area with particular energy and originality. Questioning the traditional classification of of as a preposition, he proposes a new, more semantically based, approach to the analysis of noun phrases traditionally said to contain postmodification of the head with an of-phrase, e.g. the horns of the bull. This chapter reviews Sinclair’s development as a grammarian, examines the arguments of the 1991 paper and suggests that while his work has been radically transformative in linguistics, it should not indefinitely avoid engaging with the problem of how best to represent what it is that we know when we say we know a language.