Art. 83 EPC requires that the European patent application discloses the invention in a manner sufficiently clear and complete for it to be carried out by a person skilled in the art. In certain circumstances, for example when undefined parameters are used in the claims and no details of the measuring methods are supplied, the question arises whether there is a problem with respect to Art. 83 or Art. 84 EPC. The answer is important, because Art. 83 EPC is a ground for opposition, whereas Art. 84 EPC is not. For a detailed analysis of the case law on Art. 83 EPC and clarity of claims see chapter II.C.7.2.