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According to Art. 3(4)(iii) PCT, an international application must comply with "the prescribed requirement of unity of invention". In the EPC, the corresponding requirements for unity are set out 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 (corresponding to Annex B, paragraph (b) PCT Administrative Instructions, as in force from 1.7.2008), which states that principle and noted that the PCT Administrative Instructions are 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) and (e) PCT (see G 1/89, OJ 1991, 155; in the PCT Regulations, as in force from 1.4.2005, the reference to the "three-member board" has been replaced with that to the "review body").
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.
Neither the PCT itself nor the Rules under the PCT contain provisions indicating how to decide whether or not an international application complies with the prescribed requirement of unity of invention. However, in the PCT Search Guidelines it is stated, inter alia, that lack of unity of invention 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 (G 1/89, OJ 1991, 155).