Facet design is enjoying a revival, in particular because of the interest of British environmental researchers (e.g. Canter et al.). However, if we examine the actual methodology that lies behind it, we find disappointingly little that can be useful for any researcher who is at the beginning of his research project. Admittedly, once the 'mapping sentence' is generated and put into practice in the construction of a measuring instrument, there is an abundance of (statistical) techniques that can be used. But it is that very first step of generating the mapping sentence which is in need of clarification and elaboration. Various approaches to generation will be discussed, each with its own prescriptions and hints for users. The scarce literature on how to design facets will be reviewed and augmented by insights and suggestions from such diverse sources as philosophical concept analysis, logic, taxonomy and commonsense. A case-study of development of one particular mapping sentence will be presented to serve as a running illustration of the pitfalls and possibilities likely to be met in designing facet designs.