It is a generally recognised rule of international law that when interpreting international treaties ancillary use may be made of material relating to their genesis. Under Art. 32 of the Vienna Convention, recourse may be had to supplementary means of interpretation, particularly the preparatory work for a treaty and the circumstances in which it was concluded, in order to confirm the meaning arrived at by application of Art. 31 or to determine the meaning when application of Art. 31(a) leaves the meaning ambiguous or obscure, or (b) produces a meaning which is obviously nonsensical or unreasonable (T 128/82, OJ 1984,164; see also G 2/07, OJ 2012, 130; G 1/08, OJ 2012, 206, point 4.3 of the Reasons; G 2/12 and G 2/13).

In G 2/12 and G 2/13 the Enlarged Board stated that the preparatory work ("travaux préparatoires") and the circumstances of the conclusion of the EPC serve only as supplementary sources of evidence to confirm the result of the interpretation or if no reasonable meaning can be determined by applying the general rule of interpretation (Art. 32 Vienna Convention).

In J 8/82 (OJ 1984, 155) the board noted however that it was well recognised that Art. 31 and Art. 32 of the Vienna Convention on the interpretation of treaties merely codified existing public international law. In J 4/91 (OJ 1992, 402), for example, the Legal Board of Appeal drew on historical material relating to the EPC 1973 to support its view, arrived at from a teleological and systematic interpretation of the relevant provisions, regarding the additional period for paying renewal fees. G 1/98 (OJ 2000, 111) discussed the purpose of Art. 53(b) EPC 1973, its relationship to other international treaties and legal texts, and its legislative history. In G 3/98 and G 2/99 (OJ 2001, 62 and 83), the Enlarged Board reached its conclusions on Art. 55(1) EPC 1973 after interpreting the wording and considering also the legislator's intention and the aspects of systematic, historical and dynamic interpretation.