In accordance with G 1/95 (OJ 1996, 615), alleged public prior use does not constitute a ground for opposition under Art. 100(a) EPC but is a fact cited to substantiate such a ground (T 190/05).

An allegation of public prior use will only meet the third requirement of R. 76(2)(c) EPC if it is sufficiently substantiated. It is established case law (e.g. T 328/87, OJ 1992, 701; T 538/89; T 988/91; T 541/92; T 28/93; T 927/98; T 900/99, T 1022/99; T 190/05; T 25/08; T 1856/11) that, when an opposition ground is based on an allegation of public prior use, the requirements of R. 76(2)c) EPC are fulfilled if the notice of opposition indicates, within the opposition period, the facts which make it possible to determine

The notice of opposition must also indicate the arguments and evidence in support of the alleged prior use. In T 1856/11 the board took the information contained in the supporting documents filed during the opposition period into account for establishing the required details.

An abstract indication of the subject-matter of the prior use is generally insufficient. Instead, the opponent must compare its features with those of the contested claim and demonstrate the technical parallels between them (see e.g. T 28/93, T 25/08, T 426/08). This can, however, be dispensed with in straightforward cases in which the facts are immediately self-evident to the average skilled person (T 1069/96, T 25/08, T 426/08).

In T 538/89 the board stressed that the evidence offered in support of public prior use could be submitted after expiry of the period for opposition since R. 55(c) EPC 1973 (R. 76(2)(c) EPC) only required that it be indicated (see also e.g. T 234/86, OJ 1989, 79; T 752/95; T 249/98; T 1022/99 and T 25/08). According to the board, nomination of a witness for subsequent examination is deemed to constitute indication of evidence (see also T 28/93, T 988/93 and T 241/99). In T 1553/07 the board held that the naming of a witness to be heard later is to be regarded as a sufficient indication of evidence as long as it is clear for what assertions of facts the witness is being named. There is no need also to indicate what the witness is able to say about the alleged facts.

A distinction must be made between examining the admissibility of the opposition and its substantive merit. The information relating to the "when", the "what" and the "how" of the circumstances surrounding prior use is all that the patent proprietor and the opposition division need in order to understand the case of the opponent and for the opposition to be admissible. The rest, i.e. whether the alleged facts were or would be sufficiently proven through additional filings in the course of the opposition proceedings, is a question of allowability in terms of substantive law. Thus, establishing that the alleged prior use was indeed public has little bearing on the admissibility of the opposition, but could be significant for assessing its allowability in terms of substantive law (see T 406/92, T 786/95, T 1022/99; cf. also T 533/94, T 534/94, T 1553/07, T 25/08, T 1856/11).

Whether sufficient details of the "when, what and how" have been submitted is also examined by the boards in order to establish whether (and, if so, at what point) a late-filed prior use has been sufficiently substantiated to be considered in the proceedings (see e.g. T 441/91, T 97/92, T 611/97, T 460/13, T 1955/13; on the admission of late-filed submissions, see in this chapter IV.C.4.). Similarly, evidence offered can be taken only if the facts it is intended to prove have already been sufficiently substantiated (see e.g. T 1271/06, citing T 297/00).

Several decisions note that specification of the circumstances of the act of use also requires that the opponent indicates "to whom" the subject-matter was made available (see e.g. T 522/94, OJ 1998, 421; T 339/01; T 1553/07; T 2010/08; T 1927/08). In T 241/99, for instance, the board considered it insufficient for the purposes of R. 55(c) EPC 1973 (R. 76(2)(c) EPC) that, in the event of a purported sale to a small, closed group of customers, these were identified only by a coded indication (list of numbers). This was different from the sale of mass-produced goods to anonymous buyers and resembled rather the case of a single sale, for which the requirements of R. 76(2)(c) EPC were met only if the name and address of the sole buyer were indicated within the opposition period.

Citing this decision, the board in T 55/01 noted in the context of the applicable standard of proof that case law had taken into account that cases of mass-produced consumer goods which are widely advertised and offered for sale to customers who often remained anonymous may require a different treatment. In the case at issue no allegations concerning the identity of the purchasers had been made and no proof concerning this matter produced. The sale of the mass-produced product on the market and the distribution of the corresponding service manual was nevertheless considered as proven.

The evaluation of evidence in cases of alleged public prior use is dealt with in detail in chapter III.G.4.3.2.