The boards have repeatedly pointed out that the closest prior art for assessing inventive step is normally a prior art document disclosing subject‑matter conceived for the same purpose or aiming at the same objective as the claimed invention and having the most relevant technical features in common, i.e. requiring the minimum of structural modifications (see in this chapter I.D.3.2.). A further criterion for the selection of the most promising starting point is the similarity of the technical problem (see in this chapter I.D.3.3.). In a number of decisions, the boards have explained how to ascertain the closest prior art constituting the easiest route for the skilled person to arrive at the claimed solution or the most promising starting point for an obvious development leading to the claimed invention (see in this chapter I.D.3.4. and I.D.3.5.).

In T 1212/01 the board held that the determination of the closest prior art is an objective and not a subjective exercise. It is made on the basis of the notional skilled man's objective comparison of the subject-matter, objectives and features of the various items of prior art leading to the identification of one such item as the closest.

The expression "closest prior art" does not mean that it must be sufficiently close to the claimed invention on an absolute basis, but only that it must be relatively closer to the claimed invention than the other prior-art disclosures (T 698/10). The closest prior art must be assessed from the skilled person's point of view on the day before the filing or priority date valid for the claimed invention (T 24/81, OJ 1983, 133; T 772/94; T 971/95; see also Guidelines G‑VII, 5.1 – November 2018 version).

As closest prior art, a "bridgehead" position should be selected, which the skilled person would have realistically taken under the "circumstances" of the claimed invention. Among these "circumstances", aspects such as the designation of the subject matter of the invention, the formulation of the original problem and the intended use and the effects to be obtained should generally be given more weight than the maximum number of identical technical features (T 870/96; see also T 66/97).

In T 1742/12 the board endorsed T 967/97 and T 21/08, in which it was found that if the skilled person had a choice of several workable routes, i.e. routes starting from different documents, which might lead to the invention, the rationale of the problem and solution approach required that the invention be assessed relative to all these possible routes, before an inventive step could be acknowledged (see also T 323/03, T 1437/09, T 308/09, T 259/15). Conversely, if the invention was obvious to the skilled person in respect of at least one of these routes, then an inventive step was lacking (see also T 558/00, T 308/09, T 1437/09, T 2418/12, T 1570/13). In T 967/97 it was further stated that, if an inventive step was to be denied, the choice of starting point needed no specific justification. The board in T 1742/12 also held that a piece of prior art may be so remote from the claimed invention, in terms of intended purpose or otherwise, that it can be argued that the skilled person could not conceivably have modified it so as to arrive at the claimed invention. Such prior art might be referred to as "unsuitable". However, this did not prohibit the consideration of an inventive step assessment starting from a piece of prior art with a different purpose (see also T 855/15, T 2304/16).

If a piece of prior art is "too remote" from an invention, it should be possible to show that the invention is not obvious to a skilled person having regard to this piece of prior art (T 855/15, T 2057/12, T 2304/16). A generically different document cannot normally be considered as a realistic starting point for the assessment of inventive step (T 870/96, T 1105/92, T 464/98).

In T 176/89 the board concluded that the closest prior art comprised two documents in combination with each other. It found that, exceptionally, the two documents had to be read in conjunction; they had the same patentee, largely the same inventors, and clearly related to the same set of tests. As a rule, however, when assessing inventive step, two documents should not be combined if, in the circumstances, their teaching is clearly contradictory (see also T 487/95).

The board in T 2579/11 ruled that there was no justification for disregarding a priority application and choosing a subsequent application as the closest prior art simply because the description in the subsequent application was by and large more detailed.

Public prior use may be used as the closest state of the art (T 1464/05).

In T 172/03 the board held that the term "state of the art" in Art. 54 EPC 1973 should be understood as "state of technology", and that "everything" in Art. 54(2) EPC 1973 is to be understood as concerning the information which is relevant to a field of technology. It could hardly be assumed that the EPC envisaged the notional person skilled in the (technological) art would take notice of everything, in all fields of human culture and regardless of its informational character.

However, the board in T 2101/12 considered that the interpretation of Art. 54(2) EPC given in T 172/03 was incorrect. According to the board in T 2101/12, the legislator would have used a different term if such meaning had indeed been intended. The wording of Art. 54(2) EPC is clear and requires 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.