Determining what constitutes common general knowledge plays an important role in the assessment of novelty, inventive step and sufficiency of disclosure. In the assessment of inventive step and sufficiency of disclosure however, the skilled person's knowledge is taken to be one and the same, see chapters I.D.8.3. "Skilled person – level of knowledge" and II.C.4. "Knowledge of skilled person relevant for assessing sufficiency of disclosure" for other decisions on common general knowledge.

According to established case law, common general knowledge is to be found in basic handbooks, monographs, encyclopaedias, textbooks and reference books. It is knowledge that an experienced person in the field in question is expected to have, or at least to be aware of, to the extent that he knows he could look it up in a book if he needed it. Statements in such works are often used as references to show what common knowledge is (T 766/91, T 234/93, T 590/94, T 671/94, T 438/97, T 1253/04, T 1641/11).

However, Art. 54(2) EPC 1973 does not limit the state of the art to written disclosure in specific documents only; rather it defines it as including "any other way" by which technical subject-matter is made available to the public. Therefore, the absence of a reference to a particular document does not mean that there is no state of the art, as this could reside solely in the relevant common general knowledge, which, again, might be in writing, i.e. in textbooks or the like, or be simply a part of the unwritten "mental furniture" of the notional "person skilled in the art" (T 939/92, OJ 1996, 309; T 329/04).

In T 766/91 the board stated that by its very nature common general knowledge could be inferred from a number of sources, and proof that something was general knowledge in a particular art did not depend on proof of any particular document being published at a particular date.

In T 786/00 the board stated that, according to established case law, when considering the question of novelty, a prior art document must be interpreted in the light of common general knowledge available at its publication date. Common general knowledge which did not exist at this date, but which only became available at a later date, could not be used to interpret such a document (cf. T 229/90, T 965/92).

In T 1117/14 the application was silent about how the claimed biodegradable implant was to be produced. The board stated that, according to established case law, the skilled person may use his common general knowledge to supplement the information contained in the application. It was convinced that the method formed part of the common general knowledge in the field. Although it had not yet made its way into textbooks or monographs, it had been published in both the scientific and the patent literature and those skilled in the art relied on such articles and patent documents.

In T 2101/12 in the board's view the most suitable starting point was common general knowledge. The common general knowledge was the non-technical process of the signing of a contract at the notary's office. The board in T 2101/12 considered that the interpretation of Art. 54(2) EPC given in T 172/03 was incorrect. It held that the wording of Art. 54(2) EPC was clear and required no interpretation. Art. 54(2) EPC itself contains no limitation according to which a non-technical process, such as the signing of a contract at the notary’s office, may not be considered state of the art.