The description should also mention any background art of which the applicant is aware, and which can be regarded as useful for understanding the invention and its relationship to the prior art; identification of documents reflecting such art, especially patent specifications, should preferably be included. This applies in particular to the background art corresponding to the first ("prior art") portion of the independent claim or claims (see F‑IV, 2.2).
In principle, when filing an application the applicant should cite in the description the closest prior art known to him. It may happen that the prior art cited by the applicant is not the closest existing for the claimed invention. Therefore, the documents cited in the application as filed do not necessarily describe the known innovations closest to the claimed invention, but may in fact constitute more distantly related prior art.
The insertion into the statement of prior art of references to documents identified subsequently, for example by the search report, is during examination may be necessary required, where necessary, to put the invention into proper perspective (see T 11/82). For instance, while the originally filed description of prior art may give the impression that the inventor has developed the invention from a certain point, the cited documents may show that certain stages in, or aspects of, this alleged development were already known. In such a case the examining division requires a reference to these documents and a brief summary of the relevant contents. The subsequent inclusion of such a summary in the description does not contravene Art. 123(2). The latter merely lays down that, if the application is amended, for example by limiting it in the light of additional information on the background art, its subject-matter must not extend beyond the content of the application as filed. But the subject-matter of the European patent application within the meaning of Art. 123(2) is to be understood – starting off from the prior art – as comprising those features which, in the framework of the disclosure required by Art. 83, relate to the invention (see also H‑IV, 2.1). In addition, relevant prior-art documents not cited in the original application may be subsequently acknowledged in the description even if these were known to the applicant at the time of filing (T 2321/08 and H‑IV, 2.2.6).
References to the prior art introduced after filing must be purely factual. Any alleged advantages of the invention must be adjusted if necessary, in the light of the prior art.
New statements of advantage are permissible provided that they do not introduce into the description matter which extends beyond the content of the application as filed (see H‑V, 2.2).
The applicant may cite documents in the application which relate to standard technical knowledge (background art neither addressing the same technical problem nor necessary to complete the disclosure of the claimed invention). Such citations typically relate to well-known tests for measuring certain parameters mentioned in the description or to the definitions of terms of established meaning that are used in the application. Usually they are not relevant for assessing the patentability of the claimed invention, unless for example they contain relevant information which the applicant does not mention in the description.
Acknowledgment of prior art relevant to the dependent claims only is generally not required. If the applicant indicates that subject-matter initially cited as prior art is only "in-house state of the art", such prior art may not be used in the assessment of novelty and inventive step (see T 654/92, Reasons 4, and T 1001/98, Reasons 3). However, it may be allowed to remain in the description, provided the fact that it is only "in-house state of the art" is made clear.
If the relevant prior art consists of another European patent application falling within the terms of Art. 54(3), this relevant prior document belongs to the state of the art for all contracting states. This is the case even if the two applications do not share any commonly designated state, or the designation of commonly designated states has been dropped (see G‑IV, 6). The fact that this document falls under Art. 54(3) must be explicitly acknowledged. Thus the public is informed that the document is not relevant to the question of inventive step (see G‑VII, 2). According to Rule 165, the above also applies to international applications designating EP, for which the filing fee pursuant to Rule 159(1)(c) has been validly paid and, where applicable, the translation into one of the official languages has been filed (Art. 153(3) and Art. 153(4)) (see G‑IV, 5.2).