The word senses in a published dictionary are a valuable resource for natural language processing and textual criticism alike. In order that they can be further exploited, their nature must be better understood. Lexicographers have always had to decide where to say a word has one sense, where two. The two studies described here look into their grounds for making distinctions. The first develops a classification scheme to describe the commonly occurring distinction types. The second examines the task of matching the usages of a word from a corpus with the senses a dictionary provides. Finally, a view of the ontological status of dictionary word senses is presented.