On the interpretation of provisions of the EPC, the EPC itself provides relatively little guidance. There is the Protocol on the Interpretation of Art. 69 EPC of 5 October 1973 as revised by the Act revising the EPC of 29 November 2000. This deals with the extent of protection under Art. 69 EPC. Otherwise, the EPC contains only regulations like the hierarchy of legal provisions (Art. 150(2) and 164(2) EPC) and the equal authenticity of the EPC in its three official language versions (Art. 177 EPC). Art. 125 EPC is not concerned with the interpretation of the EPC but serves merely as a "fill-in" in case of missing procedural provisions. The EPC itself thus contains no general rules for its interpretation, and one must look outside the terms of the Convention itself for such rules (G 2/12, G 2/13, OJ 2016, A27 and OJ 2016, A28).

As an international treaty, the EPC has to be interpreted in accordance with the rules of interpretation developed in the so-called "law of nations" or public international law. To the traditional kind of international treaty which regulates legal relations between States had to be added the treaty which directly creates and defines rights and duties for individuals and corporate bodies (G 5/83, OJ 1985, 64; G 2/12, G 2/13).

In the interpretation of international treaties which provide the legal basis for the rights and duties of individuals and corporate bodies it is necessary to pay attention to questions of harmonisation of national and international rules of law. The boards of appeal may take into consideration decisions and opinions given by national courts in interpreting the law (see G 5/83, OJ 1985, 64; see also G 2/12, G 2/13). Nevertheless, in the proceedings before the European Patent Office, such considerations do not exonerate a board of appeal from its duty as an independent judicial body to interpret and apply the EPC and to decide in last instance in patent granting matters. TRIPS provisions, like decisions of the European and International Courts of Justice and national decisions, are elements to be taken into consideration by the boards of appeal but are not binding on them (T 154/04, OJ 2008, 46).

Although the European Patent Organisation is not a party to the Vienna Convention on the Law of Treaties concluded on 23 May 1969 (hereinafter Vienna Convention), the principles of interpretation of Arts. 31 and 32 of the Vienna Convention are to be applied to the interpretation of the EPC even though its provisions do not apply to the EPC ex lege (G 5/83, G 2/08 date: 2010-02-19, OJ 2010, 456). The boards of appeal refer to legal sources outside the EPC, including, for example, Vienna Convention and the TRIPS Agreement. Thus the boards of appeal may be guided in their decisions by the provisions of other international instruments. However, they have no obligation to apply them directly (G 2/02 and G 3/02, OJ 2004, 483).