Where an assertion that something is common general knowledge is challenged, the person making the assertion must provide proof that the subject-matter in question is in fact common general knowledge (T 438/97, T 329/04, T 941/04, T 690/06, T 2132/16). In the case of any dispute as to the extent of the relevant common general knowledge this, like any other fact in contention, has to be proved, for instance by documentary or oral evidence (T 939/92, OJ 1996, 309, see also T 766/91, T 1242/04, OJ 2007, 421; T 537/90, T 329/04 and T 811/06). The proof is regularly supplied in citing literature (T 475/88). According to T 766/91 and T 919/97, evidence of general technical knowledge need be submitted only if the latter’s existence is disputed.

In ex parte case T 1090/12 the appellant drew attention to section G‑VII, 3.1 of the Guidelines (unchanged in the November 2018 version) that reads: "an assertion that something is common general knowledge need only be backed by documentary evidence (for example, a textbook) if this is contested" and submitted that this passage was binding on the boards of appeal when exercising the power of the examining division. In the case at hand, the board denied that it was obliged to provide written evidence and explained during the oral proceedings that its members knew from their work on previous cases that the features at issue were known to the person skilled in the art. It would therefore have been for the appellant to show an error in the board's appreciation, e.g. that one of the features at issue was in fact introduced into the art after the priority date of the application under appeal. There is also no general obligation on a board to provide documentary evidence for the existence of a piece of common general knowledge. In proceedings before the EPO, a board of appeal has to respect the right to be heard and to give reasons for its decision. In cases where a board refers to common general knowledge as state of the art, it is not obliged to provide documents in every conceivable case. It is also possible for a board to state what it deems to be known, and possibly where it is known from, in a way that puts the appellant in a position to try to convince the board that its findings are erroneous. Proceeding that way respects the party's right to be heard. In this respect, the request for referral of questions to the Enlarged Board of Appeal was refused.

In T 1540/14, another ex parte case, the board, after giving detailed reasons and summarising the case law on ascertaining the common general knowledge, concluded that there was not enough conclusive evidence to support the examining division's reasons for refusing the application for lack of inventive step because the features distinguishing its claim 1 from the closest prior art were obvious in view of the common general knowledge illustrated by two patent literature documents.

In ex parte case T 2101/12 the common general knowledge was the non-technical process of the signing of a contract at the notary's office. According to the appellant, the EPO has the burden of proving the content of the common general knowledge it relies on. The appellant had at no point disputed that the process mentioned by the board was indeed common general knowledge. No documentary evidence was required to prove the extent of the cited common general knowledge.

In T 1110/03 (OJ 2005, 302) the board stated that when evaluating evidence relating to the issues of novelty and inventive step it was necessary to distinguish between a document alleged to be part of the state of the art within the meaning of Art. 54(2) EPC 1973 – in the sense that the document itself was alleged to represent an instance of what had been made available to the public before the priority date of the opposed patent – and a document which was not itself part of the state of the art, but which was submitted as evidence of the state of the art or in substantiation of any other allegation of fact relevant to issues of novelty and inventive step. Only a document of the first kind could be disregarded on the sole ground that it was post-published; documents of the second kind did not stand or fall by their publication date even on issues of novelty and inventive step. Similarly a technical review article is by definition an account of the common general knowledge in the art prior to its own publication date – which could bear inter alia on the issue of enabling disclosure of a prepublished document and hence on the novelty of claimed subject-matter (T 1625/06, T 608/07, T 777/08).

In T 608/07 the respondent tried to query the relevance of D6 because it was published after the priority date of the patent in suit. The board stated that D6 related to published course material of a university and to subject-matter well known in the art for many years. Hence, albeit published after the priority date of the patent in suit, D6 provided indirect evidence for common general knowledge.

See also T 2196/15 for an example of difficulties in providing the common general knowledge.