Many decisions which use the description and drawings to interpret the claims concern the interpretation of relative, ambiguous or unclear terms or cite the description merely to confirm the most obvious interpretation of the text of a claim (e.g. T 23/86, OJ 1987, 316; T 16/87, OJ 1992, 212; T 327/87, T 121/89, T 476/89, T 565/89, T 952/90, T 439/92, T 458/96, T 717/98, T 556/02).
In decision T 50/90 the board pointed out that the description and drawings were used to interpret relative terms contained in the claims when the extent of protection had to be determined.
When the technical content of the claims and the description clearly establish how an invention operates, these features cannot be disregarded for the purposes of assessing patentability by interpreting them merely as defining an intended use (T 458/96).
In several decisions the boards stated that terms used in patent documents should be given their normal meaning in the relevant art, unless the description gives them a special meaning. The patent document may be its own dictionary (see e.g. T 311/93 and T 1321/04, T 1388/09 and point 6.3.5 below). Likewise in T 500/01, the board considered that a patent, being a legal document, may be its own dictionary. If it was intended to use a word which is known in the art to define a specific subject-matter to define a different matter, the description may give this word a special, overriding meaning by explicit definition. Consequently, a claim, the wording of which is essentially identical to a claim as originally filed, can nevertheless contravene the requirements of Art. 123(2) EPC, if it contains a feature whose definition has been amended in the description in a non-allowable way.
In T 1023/02 the invention was based on the recognition that the genes for a specific viral protein (ICP34.5) expressed in infected cells determine the ability of herpes simplex viruses to destroy central nervous system tissue. The respondent argued that a post-published document revealed the existence of an ORF-P gene coincident with but anti-sense to the ICP34.5 gene. Therefore, in view of the term "only", the claim had to be interpreted as requiring that the first method step did not interfere with the expression of this ORF-P gene. The board, however, pointed out that it was apparent from the description that the patentee had not envisaged the existence of this gene. The skilled reader would therefore, in the light of the description, not interpret the subject-matter of claim 1 in the way advocated by the respondent. Accordingly post-published knowledge of further technical details and/or complications could not justify this interpretation.