The third requirement of R. 76(2)(c) EPC for the admissibility of the opposition is the substantiation of the specified ground, i.e. an indication of the facts, evidence and arguments presented in support of it. The fulfilment of this latter requirement in respect of one of the grounds for opposition is enough to render admissible the opposition as a whole. Nowhere in the EPC is there any basis for the concept of partial admissibility of oppositions. The concept of "inadmissibility" is only applicable to the notice of opposition as a whole (T 653/99; see also T 212/97 and T 65/00).

According to T 114/95, if an opponent requests revocation of the patent in its entirety, it is sufficient to substantiate the ground(s) of opposition in respect of at least one claim of the patent for the requirements of R. 76(2)(c) EPC (R. 55(c) EPC 1973) to be met (T 926/93, OJ 1997, 447; T 1180/97, T 1900/07; see also in this chapter IV.C.3.2.).

The question whether the requirements in R. 76(2)(c) EPC are met must be determined on expiry of the nine-month opposition period (R. 77(1) EPC). A notice objectively complying with the requirements on the relevant date is not rendered inadmissible where the opponent later cites prior art not mentioned in it to support his line of argument (T 1019/92; see also T 104/06).

In T 222/85 (OJ 1988, 128) the board held that the third requirement was only satisfied if the contents of the notice of opposition were sufficient for the opponent's case to be properly understood on an objective basis, from the point of view of a reasonably skilled person in the art to which the opposed patent relates. The board reasoned that the purpose of the third requirement of R. 55(c) EPC 1973 (R. 76(2)(c) EPC), in combination with the first two requirements, was to ensure that the notice of opposition set out the opponent's case sufficiently so that both the patentee and the opposition division knew what that case was. Whereas the requirements of R. 55(a) and (b) EPC 1973 (R. 76(2)(a) and (b) EPC) and the requirements (1) and (2) of R. 55(c) EPC 1973 could be considered as formal in nature, requirement (3) of R. 55(c) EPC 1973, in combination with Art. 99(1) EPC, was substantive in nature, and called for reasoning which went to the merits of the opponent's case. A well-drafted opposition ought to contain reasoning that was full but concise. And in general, the less reasoning that a notice of opposition contained, the greater the risk that it would be rejected as inadmissible (similarly T 925/91, OJ 1995, 469; see also T 2/89, OJ 1991, 51; T 448/89, OJ 1992, 361; T 545/91, T 204/91). According to the board, the question whether a particular notice of opposition meets the minimum substantive requirements of Art. 99(1) EPC 1973 and R. 55(c) EPC 1973 could only be decided in the context of the particular case (since various relevant factors, such as the complexity of the issues raised, vary from case to case) – see also e.g. T 534/98, T 1097/98, T 934/99, T 426/08. In T 623/18 (see also the summary in this chapter IV.C.2.2.8 b)) the board qualified the principles established in T 222/85 by emphasising inter alia that there was no basis for making the admissibility of the opposition dependent on substantive questions, such as whether an objection relates merely to a lack of clarity.

In T 204/91 the board stated in particular that the term "indication" in R. 55(c) EPC 1973 (R. 76(2)(c) EPC) had to be construed as requiring more than a mere hint at a number of possible attacks upon the patent and at the likely support for each such possible attack, which indication or hint might then be augmented by the subsequent late filing, possibly even at the appeal stage, of further evidence, arguments or other matter – even of fresh grounds of objection. Rather, the scope and depth of "indication" needed to be such as to enable the patentee and the opposition division to see clearly just what attack was being mounted against the patent, and what evidential support was being adduced for that attack. In other words, the patentee and the opposition division had to be put in a position of understanding clearly the nature of the objection submitted as well as the evidence and arguments in its support. This required the elaboration of the relevant circumstances of the case to such an extent that the patentee and the opposition division were able to form a definitive opinion on at least one ground for opposition raised, without the need to make further investigations (see T 453/87 and T 279/88; see further e.g. T 1069/96 and T 426/08).

The fact that a patentee must be able to understand, without undue burden, the case made against his patent in the notice of opposition does not, however, exclude the possibility that he may have to undertake a certain amount of interpretation (T 199/92; see also T 1553/07, T 265/16; see however T 1082/00, which highlights the limits of this principle and refers to T 204/91).

In T 934/99 the board stated that R. 55(c) EPC 1973 (R. 76(2)(c) EPC) did not imply the requirement of a logical line of reasoning in the sense that the arguments brought forward in the notice of opposition had to be cogent or convincing. Rather, the criterion was whether the arguments presented were relevant and, where necessary as the result of a reasonable interpretative effort, specific enough to allow a person skilled in the art to form a reasoned opinion of whether the line of reasoning on which the opponent apparently relies was (logically) correct ("convincing") or not (i.e. wrong).

The sufficiency of the notice of opposition has to be distinguished from the strength of the opponent's case. On the one hand, an unconvincing ground for opposition might have been clearly presented and argued. Conversely, a deficient submission may be rejected as inadmissible even though, if properly drafted, it would have succeeded (T 222/85, OJ 1988, 128; T 621/91, T 3/95, T 1069/96, T 1856/11). Assessing the evidence is part of the process of ascertaining whether the opposition is well founded in substance (T 234/86, OJ 1989, 79). Thus, in T 353/06 the board identified the appellants' arguments for the inadmissibility of the opposition as concerning the corroborative value of the facts and evidence, rather than a failure to indicate such facts and evidence in support of the grounds of opposition.

In the same vein, in T 65/00 the board held that it was of no importance for admissibility whether the relevance of the arguments brought forward, or their correctness, was sufficient to revoke the patent. In the case at issue it was therefore irrelevant whether the arguments brought forward by the opponent referred to Art. 84 or Art. 83 EPC 1973. For admissibility it was sufficient that the arguments were such that an arguable case was established. See also T 623/18.

Similarly, the board in T 1194/07 referred to the established case law, according to which the indication of facts, evidence and arguments need not be conclusive or correct. That is an issue of merit. However, the board also emphasised that the merits of a case can only be assessed in a meaningful manner if the case has been stated fully, by an adequate indication of the relevant facts, evidence and arguments. Key elements linking the cited prior art to the claim or relating to their validity as evidence, as well as crucial arguments informing the reader why the skilled person might have considered combining the features of such prior art with those of the accompanying evidence, should not be missing in the notice's chain of logic leading from the prior art to the invention. The omission of key facts and elements in the notice's reasoning may amount to little more than speculation or an outline of a possible case against the patent (see also T 1856/11).

In T 344/88 the opponent had given the wrong number for a patent specification which it had cited against the patent in suit. The board considered whether R. 55(c) EPC 1973 (R. 76(2)(c) EPC) was contravened if the mistake was not corrected until after expiry of the opposition period. While the patent cited bore no relation to the invention under consideration, the notice of opposition contained enough information to identify the actual patent intended and so the board allowed the number to be corrected. Given such a detailed submission of facts, it would have been taking formal requirements too far to reject the opposition simply because the wrong number had been given for a cited patent specification.

In T 426/08, the board considered that the content of evidence merely referred to within the opposition period but not produced until after it could not, for the purpose of examining admissibility, be taken into account either as an indication of facts or evidence or as proof of facts.

In T 1022/09 the opposition division had confused the criteria for admissibility of late-filed documents with those for documents filed with and referred to in an admissible notice of opposition. According to the board, the content and publication date of the documents filed with the notice of opposition did not come into play for the question of admissibility. Since their filing was substantiated, they were automatically admitted to the opposition proceedings.