Most organisations seem to use a classification system (or taxonomy) of some sort, for instance for safety classification, and much time is spent developing and using such taxonomies. Importantly, decisions may be made on the basis of the taxonomy and associated database outputs (or it may be that much time is spent on development and use, but little happens as a result). There is therefore a risk of time and money spent unnecessarily, with associated opportunity costs. Still, taxonomies are a requirement in all sorts of areas, and several things should be kept in mind when designing and evaluating a taxonomy. This posts introduces twelve properties of effective classification systems.