Article 84 of the European Patent Convention requires claims to be clear. Article 69 permits one to refer to the specification to interpret a claim, unless the claim is so clearly bad (for example by containing mutually contradictory requirements) that "interpretation" was inappropriate. In Water-soluble cellulose ether/Aqualon[1] it was pointed out that whereas if one were referring to absolute values in a claim then if it was not obvious the test used to determine those values might have to be defined, no such requirement should exist for relative terms such as "water soluble". On the other hand relative terms themselves may be found to lack clarity. Thus in Lower alkyl/Nihon Nohyako[2] it was held that the term "lower alkyl" used to define certain substituents of a pyrazole derivative which was being claimed per se was lacking in clarity in the absence of any definition of the term in the specification. Having looked at text book uses of the term, the board felt that it had different meanings depending upon the context in which it was used and that there was not a sufficient reference point to define that context in the present case.

Defining an invention in functional terms has long been held to be permissible. However, recent case law has held that if an invention is defined in functional terms, in order to meet the requirement of sufficiency in the disclosure, it may be necessary for the specification to contain not only a sufficient number of examples, but also a "technical concept fit for generalization". This would enable the skilled person to achieve the envisaged result without difficulty within the whole ambit of the claim containing the 'functional' definition."[3] There may however be situations in which there is a risk that some of the national courts, before which the patent in question might be litigated would take a dim view of the claims allowed by the EPO. One such situation is cases in which the claim is characterized by too much functionality or statements of desiderata.[4]