According to Art. 3(4)(iii) PCT, an international application must comply with "the prescribed requirement of unity of invention", the details of which are set out in R. 13 PCT. In the EPC, the requirements for unity are laid down in Art. 82 EPC and R. 44 EPC.

In W 6/97 it was held that the determination of unity of invention must be made on the basis of the contents of the claims as interpreted in the light of the description and of the drawings, if applicable. The board referred to Annex B, Part 1(b) of the PCT Administrative Instructions (cf. now Annex B, para. (b) PCT Administrative Instructions, as in force from 1.1.2019), which stated that principle, and noted that the PCT Administrative Instructions were binding not only on the ISA but also on the board of appeal (when formerly) acting as the "three‑member board" according to R. 40.2(c) PCT and R. 40.2(e) PCT (see G 1/89, OJ 1991, 155; since 1.4.2005, the references in the PCT Regulations to "three-member board" have been replaced by "review body" – see also in this chapter II.B.1.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").

In W 39/90 the board observed that it was not the formal choice of words or form of reference, but the actual content of the claims which established technical relationships between the subject-matter of different claims, and which was thus decisive for the question of unity. In W 33/92 the board emphasised that R. 13.1 PCT did not require the link between the subject-matter of the two independent claims to be expressly stated in their wording. All that was required was that there be a single inventive concept.

In G 1/89 (OJ 1991, 155), the Enlarged Board of Appeal pointed out that neither in the PCT itself nor in the Rules under the PCT were there provisions indicating how to decide whether or not an international application complies with the prescribed requirement of unity of invention. However, the PCT Search Guidelines stated, inter alia, that lack of unity may be directly evident "a priori", that is before considering the claims in relation to any prior art, or may only become apparent "a posteriori", that is after taking prior art into consideration (see now para. GL/ISPE 10.03 PCT International Search and Preliminary Examination Guidelines as in force from 1.1.2019). See also next section.