R. 13.2 PCT defines the method for determining whether the unity requirement is satisfied in respect of a group of inventions claimed in an international application: "Where a group of inventions is claimed in one and the same international application, the requirement of unity of invention referred to in R. 13.1 PCT shall be fulfilled only when there is a technical relationship among those inventions involving one or more of the same or corresponding special technical features. The expression "special technical features" shall mean those technical features that define a contribution which each of the claimed inventions, considered as a whole, makes over the prior art" (cf. R. 44(1) EPC).

According to the case law, once such a single, i.e. common, concept was established, it was necessary to consider whether or not the same could contribute to the inventiveness of the various subject-matters claimed in the case. Nothing indicated that this concept was known or belonged to the general knowledge of the man skilled in the art. Since the search examiner did not indicate that the documents cited in the case could exclude such a contribution, it could not be assumed that this could not be the case (W 17/89; see W 6/90, OJ 1991, 438). In view of this, a single inventive concept had to be attributed to all claims linking all their subject‑matters (see W 22/91).

In W 6/90 (OJ 1991, 438; see also in this chapter II.B.5.1. above) the board noted inter alia that R. 13.1 PCT also stipulated that the single general concept had to be inventive. Even with a given single general concept there was lack of unity if the concept had no inventive character. The boards of appeal have confirmed the above decision on several occasions (see e.g. W 31/91, W 29/92, W 34/92, W 45/92, W 8/93 and W 6/97).

In W 48/90 and W 50/90 the board noted that as far as chemical compounds were concerned, unity of invention was no mere question of the respective structural features, but had to be decided taking into account the technical problem to be solved and whether or not the respective compounds contributed to the solution found.

In W 45/92 the board stated that the term "inventive" could not be interpreted as a requirement for the common part to be inventive per se and therefore claimable as such. The investigation in this respect should concentrate on whether or not all or some of such features could contribute to the later detailed consideration of the inventive steps. Only if the state of the art or common general knowledge which had already been taught showed, beyond reasonable doubt, that this was not possible in the circumstances, should non-unity be declared. It further held that the term "same or corresponding special technical features" recognised that the features should define a contribution to the invention vis‑à‑vis the prior art. As the features in question were also part of the most relevant state of the art, where they were set out in an identical manner, they could not provide such a contribution. Thus, the various inventions, if any, had to lie in the unshared specific characterising features in each case. Since the claims contained no other common feature between them in the characterising part, this confirmed the conclusion of lack of unity (see W 32/92, OJ 1994, 239).

In W 38/92 the board confirmed W 6/90 (see above) and noted that the common features in the claim groups represented the "sole concept" linking the various subject-matters. The board stated that the remaining question was whether or not any of these specific features in the claim groups or their combination could make a contribution to the inventive steps relating to each claimed subject-matter; if so, this would render the said concept linking the various subject-matters inventive. The board observed that the new R. 13 PCT also required such a contribution by the suggested common "special technical features" over the prior art, i.e. that more than novelty be provided. In T 94/91 the board made it clear that the general inventive concept could not be equated with the features cited in a claim or in a particular combination of claims. What should be considered was the inventive concept as defined in the claims with due regard to the description and any drawings (see W 2/95).

In W 9/93 the board noted that the intermediate compounds were known, so that these products could not serve as a common inventive concept, and that a technical feature contained in the process claims was also known. The board pointed out that a technical feature which already formed part of the state of the art could not, by definition, make a contribution over the prior art and did not, therefore, qualify as a unifying element within the meaning of R. 13.1 PCT. According to R. 13.2 PCT, as in force from 1.7.1992, an international application could relate to a group of inventions if there was a "technical relationship" among those inventions involving one or more of the same or corresponding "special technical features", i.e. such technical features that defined a contribution which each of the claimed inventions made over the prior art. R. 13.1 PCT did not simply require some link between a group of inventions claimed in an international application, but a common inventive concept. This meant that there must be either a common technical problem or at least, if there was more than one technical problem, there must be one single technical concept behind the solutions of these different problems.

In T 957/96 the application described a number of processes which did not have any process step in common. According to the board, the decisive fact was that all the processes claimed shared a common technical feature, namely the use of the substantially pure regioisomer, which was essential for solving the technical problem addressed by the application. It was this use of the intermediate compound which formed the common "inventive" concept of all the claimed process variants. In other words, this feature constituted a special technical feature which defined the contribution that the claimed invention made over the prior art as required by R. 30(1) EPC 1973 (see now R. 44(1) EPC).

In W 11/99 (OJ 2000, 186) the board held that if, in an international application, there are claims directed to products and to a process for the manufacture thereof, it could not be assumed that there were no corresponding special technical features within the meaning of R. 13.2 PCT simply because the process could also be used to manufacture other products. The board interpreted the requirement for the presence of "corresponding special technical features" under R. 13.2 PCT in cases where a manufacturing process and products were being claimed in the same application to mean that they could usually be assumed to be present where the production process is new and actually suited to making the claimed products accessible (where appropriate in addition to further products). In such cases the board regards the process as being "specially adapted" to the manufacture of the claimed products. A narrower interpretation of the terms "specially adapted" and "corresponding special technical features" would not fulfil the legislative purpose of Art. 34(3) PCT and associated R. 13.1 PCT, which, in the board's view, was the same as that of Art. 82 EPC 1973, namely to prevent subject‑matters which were not interconnected being claimed in one and the same application. This interpretation accorded with Annex B to the PCT Administrative Instructions mentioned in the PCT Guideline III‑7.2 (July 1998 version), where it is stated in Part 1, item (e), that a process is specially adapted for the manufacture of a product if it inherently results in the product (see now Annex B, para. (e) PCT Administrative Instructions, as in force from 1.1.2019). If this condition was met, it was irrelevant whether other products could be obtained using the process.

In T 106/06 the board referred to the established case law of the boards of appeal according to which a manufacturing process and its resulting products are considered as unitary subject matter (e.g. W 2/95 and W 11/99). The board gave consideration to the question whether the fact that claim 1 did not relate to a manufacturing process in the usual sense which may be characterised in that the specific end products are envisaged at the outset of the process, but to a process of isolating genes which, in view of its set up, has resemblance to a screening process which, in turn, may be characterised in that the final product is not known at the outset, should have an influence on the assessment of unity. The board held that the decisive question was whether the product had actually been produced by the process and not whether it had been known at the start of it. Therefore, there was no difference in the assessment of unity between a manufacturing process and a screening process and their resulting products.

In W 18/01 the board pointed out that it followed from the definitions in the Regulations under the PCT and the PCT International Preliminary Examination Guidelines (which are binding on the EPO; see G 1/89, OJ 1991, 155, point 6 of the Reasons) that it was normally not sufficient merely to define and examine the common core of a group of inventions, for example by indicating the features which were specified in all the claims defining the inventions of the group. R. 13.2 PCT required an examination of the technical relationship among the inventions in a group. Such a relationship could exist even if the inventions did not involve the same technical features, provided they involved corresponding special technical features. The definition of "special technical features" given in R. 13.2 PCT required an analysis of the contribution which each of the claimed inventions made over the prior art. This first entailed analysing which of the features distinguished the claimed inventions from the cited prior art before their contribution could be examined in the light of the description, in particular the problems solved and the effects achieved by the claimed inventions.