Nominal classification remains a fascinating topic but in order to make further progress we need greater clarity of definition and analysis. Taking a Canonical Typology approach, we use canonical gender as an ideal against which we can measure the actual gender systems we find in the languages of the world. Building on previous work on canonical morphosyntactic features, particularly on how they intersect with canonical parts of speech, we establish the distinctiveness of gender, reflected in the Canonical Gender Principle: In a canonical gender system, each noun has a single gender value. We develop three criteria associated with this principle, which together ensure that canonically a noun has exactly one gender value; we give examples of non-canonicity for each criterion, thus gradually building the typology. This is the essential groundwork for a comprehensive typology of nominal classification: the Canonical Typological approach allows us to tease apart clusterings of properties and to characterize individual properties with respect to a canonical ideal, rather than requiring us to treat the entire system as belonging to a single type. This approach is designed to facilitate comparisons of different noun classification systems across languages.