The requirement of unity of invention needs to be assessed only if a group of inventions is claimed. A group of inventions may be formed, for example, by a plurality of independent claims in the same or in different categories, or a plurality of alternative inventions embodiments defined within a single independent claim (see also F‑IV, 3.7).
If a group of inventions is claimed, the requirement that the inventions in this group are so linked as to form a single general concept (Art. 82) is fulfilled only if there is a technical relationship among the claimed inventions involving one or more of the same or corresponding special technical features.
The term "special" means that the features in question define the contribution that the invention considered as a whole makes over the prior art in terms of novelty and inventive step.
The term "same" means that the special technical features are identical or define an identical chemical structure.
The term "corresponding" means that the special technical features achieve the same technical effect or solve the same technical problem. Correspondence may be found for example in alternative solutions, or interrelated features, e.g. the interaction between a plug and a socket causing a releasable electrical connection, or in a causal relationship such as a step in a manufacturing process that causes a certain structural feature in a product. For example, an application might include two sets of claims, one comprising a metal spring, and another comprising a block of rubber. The metal spring and block of rubber may be considered to be corresponding technical features as they both achieve the same technical effect of resilience.
In contrast, features that are not shared, i.e. the rich varieties in features that only appear in some but not in other claims, cannot be part of the single general inventive concept (W 45/92).
Therefore a substantive assessment of unity of invention requires
Any ground for lack of unity must show absence of the technical relationship required by Rule 44(1), i.e. either that no common matter is present among the claimed inventions, or that the common matter is not "special".
For example, a lack of unity may arise between the dependent claims if an independent claim provides the only common matter linking them or the only common matter linking two alternatives defined within a single dependent claim, and the common matter does not comprise any features that make a technical contribution over the prior art in terms of novelty or inventive step, i.e. is not "special".