Article 69 of the European Patent Convention provides that "the extent of the protection conferred by European patents ... shall be determined by the terms of the claims. Nevertheless the description and drawings shall be used to interpret the claims". A protocol to the Convention attempts to define what this means. The protocol reads as follows:

In 1982, the House of Lords in Catnic Components Limited v. Hill and Smith, a case where the interpretation of the Convention was not an issue, adopted a principle of "purposive construction" for the interpretation of patent claims. Under tests laid out in the subsequent case of Improver v. Remington a patent claim will be held to be infringed not only by what falls in its literal wording but also by a variant thereof:

There has been an ongoing debate in recent years as to whether the purposive construction test in the Catnic case in fact complies with the protocol to Article 69 of the European Patent Convention and there is at least one first instance case which held that it does not. The issue now seems to have been settled, at least for the time being, by the decision in the case of Kastner v. Rizla Limited in which the Court of Appeal stated definitively that in its view the Catnic approach was the appropriate "middle way" contemplated by the protocol of Article 69 of the European Patent Convention and that until the House of Lords ruled otherwise the Catnic approach was to be followed. Somewhat surprisingly, in view of the fact that the Catnic approach had generally been regarded by commentators as being narrower than what they thought the protocol to Article 69 ought to mean, the Court of Appeal went on to reverse a finding of non-infringement in the case before it to hold that when applying the Catnic approach there was in fact infringement in a case where two elements of a nine-element apparatus differed from what was recited in the patent claim.