Very often in linguistics, it is simply not possible to provide a classical definition with necessary and sufficient conditions for our categories. This is the case for most (perhaps all?) linguistic categories. Even basic categories such as parts of speech are not entirely clearly defined. In fact, Langacker (1987) takes that as a sign that we should re-think our whole linguistic ideas. But how can we then correctly annotate our data as a corpus linguist? Well, that is where inter-annotator agreement comes into play.