R. 44(1) EPC and R. 13.2 PCT also apply where a single claim defines alternatives ("Markush claim"). According to R. 44(2) EPC and R. 13.3 PCT, the determination of whether a group of inventions is so closely linked as to form a single general inventive concept shall be made without regard to whether the inventions are claimed in separate claims, or as alternatives within a single claim (see also W 35/91).

In considering whether the ISA had substantiated its findings of non-unity, the board in W 3/94 (OJ 1995, 775) referred to the section on the "Markush practice" in Annex B, Part 1(f) of the Administrative Instructions under the PCT, as in force from 1.7.1992 (see now Annex B, para. (f) PCT Administrative Instructions, as in force from 1.1.2019). Here it is stated that the requirement of a technical interrelationship and the same or corresponding special technical features as defined in R. 13.2 PCT shall be considered met when the alternatives are of a similar nature. The section then specifies when alternatives are to be regarded as "of a similar nature."

In W 1/94, the EPO acting as ISA had found that the compounds according to the "inventions" it had identified did not have a new structural element. Nevertheless the board concluded that the absence of such an element did not automatically destroy the unity of the invention. On the contrary, it was clear from the PCT Administrative Instructions that a technical relationship must be recognised for a group of alternative chemical compounds, such as all the compounds having a common property or activity, and such a common activity existed in the case in point. The new use could therefore form the general inventive concept linking the subject-matters of the claims in accordance with R. 13.1 PCT and had to be treated as a functional characteristic linking the compounds claimed.

In W 6/95 the board referred to the PCT Administrative Instructions, binding not only on the ISA but also on the board of appeal acting as the "three-member board" according to R. 40.2(c) PCT (see G 1/89, OJ 1991, 155; since 1.4.2005, the references in the PCT Regulations to "three-member board" have been replaced by "review body" – see also in this chapter II.B.1. "Introduction"). The board noted that it followed from Annex B, Part 1(f)(i) PCT Administrative Instructions (now part of Annex B, para. (f) PCT Administrative Instructions, as in force from 1.1.2019) that in order to establish unity of invention "a posteriori", it was not sufficient for all alternatives of chemical compounds covered by a Markush claim to have a common property or activity, i.e. be suitable for solving a common technical problem, because according to item (B)(1) they had, in addition, to have a "significant structural element" in common in order for the alternatives to form unity. The board made it clear that it was, however, not in agreement with the explanation given in Annex B, Part 1(f)(ii) PCT Administrative Instructions to assume that the said "significant structural element" had to be novel per se. Rather, this expression meant that in relation to the said common property or activity there had to be a common part of the chemical structure which distinguished the claimed compounds from known compounds having the same property or activity (see also W 6/97).

In T 169/96 the board noted that the fact that claim 1 also comprised a known compound, not covered by claim 2, was of no relevance to the question of unity, because R. 30(b) EPC 1973 (in the version as in force until 31.5.91; also in the version in force thereafter) did not require that there must be a common concept unifying different "means" according to it. It was not relevant here that the three different classes of chemical compounds identified by the examining division comprised completely different chemical structures of a residue. The board observed that Chapter C‑III, 7.4a of the then applicable EPO Guidelines, which had been relied on by the examining division, as well as Annex B, Part 1(f) PCT Administrative Instructions (see now Annex B, para. (f) PCT Administrative Instructions as in force from 1.1.2019), concerning Markush‑type claims, rightly stated that the said significant structural element might consist of a combination of individual components linked together. It was not stated there that the combination of individual structural elements had to be novel per se, nor did such a requirement follow from Art. 82 EPC 1973. Rather it followed from that guideline that this expression meant that in relation to the said common property or activity there had to be a common part of the chemical structure which distinguished the claimed compounds from known compounds having the same property or activity (see now Guidelines F‑V, 2.2.2.2 – November 2018 version).

In W 4/96 (OJ 1997, 552) the board noted that the requirement of a technical relationship as defined in R. 13.2 PCT, first sentence, PCT might be met when all claimed alternatives belong to a class of compounds which might be expected to behave in the same way in the context of the claimed inventions ("Markush claims''). The technical relationship involved those common special technical features that defined a contribution over the state of the art (R. 13.2, second sentence, PCT; see also W 6/96, W 1/10). However, such contribution could not be recognised on the basis of this expectation if members of the class had already been shown in the prior art to behave in the manner disclosed in the application. The board added that if at least one Markush alternative was not novel over the prior art, the question of unity had to be reconsidered by the examiner, i.e. a posteriori (Annex B, Part l (f)(v) PCT Administrative Instructions in conjunction with PCT Search Guidelines; see now Annex B PCT Administrative Instructions, as in force from 1.1.2019, also PCT International Search and Preliminary Examination Guidelines, as in force from 1.1.2019).

The board also held that an objection of lack of unity could be raised a posteriori with regard to all categories of grouping of alternatives of chemical compounds. An a posteriori assessment of unity had to be made in all circumstances in the same way, since the legal requirement of R. 13 PCT was the same for all cases. Therefore, the a posteriori assessment of unity in cases of a functional relationship within a group of compounds had to be made in a manner corresponding to the assessment in cases of a structural relationship. This was appropriate for compounds related by their structure as well as for compounds related by their function. The structural relationship within the meaning of Annex B, Part l (f)(i)(B)(1) and the relationship within the meaning of Annex B, Part l (f)(i)(B)(2) and (iii) served the same purpose: both tests were intended to show whether the alternative compounds were of a similar nature, as was said in the introductory part of Annex B, Part l(f)(i). These tests were only tools in order to determine whether there was unity of invention as defined in R. 13 PCT.

Claim 1 in W 34/07 was directed to compositions for the coating of ceramic supports comprising micronised glass frits and/or nanosised zirconium hydroxide. The board concluded that the two claimed alternatives neither had in common express features not known in combination from the prior art nor represented two solutions to the same technical problem. The effects achieved by using either micronised glass or nanosised zirconium hydroxide were different and unrelated, although they might lead to "best results", i.e. when using a composition comprising both components.