2. Requirement of unity of invention 

A European patent application must relate to one invention only or relate to a group of inventions which must be so linked as to form a single general inventive concept (see also B‑VII, 1).

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, a plurality of alternative inventions defined within a single independent claim (see also F‑IV, 3.7) or a plurality of dependent claims where the independent claim is either not novel or not inventive.

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 at hand" in terms of novelty and inventive step. The "prior art at hand", i.e. the prior art relied upon in the non-unity assessment, may vary depending on the stage of proceedings (see F‑V, 3).

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. features that only appear in some but not in other claims, cannot be part of the single general inventive concept.