The unity principle, also known as the principle of "one application for one invention", means that one patent application shall be limited to one invention. The unity principle is a fundamental principle for patent applications, and in addition to state or regional patent laws in the world, PCT (Patent Cooperation Treaty) defines this principle.

It becomes a universal principle for a number of reasons. The first is economic consideration. The patent examination organization requires the applicants to pay certain fees to maintain the normal operation of the organization and enable it to complete the search and examination of patent applications. Typically, the fee paid by an applicant is based on the number of patent applications. If the number of inventions included in a patent application is not limited, an applicant may only pay the fee for a patent application, but request to examine a number of inventions, leading to heavy economic burden of the patent examination organization. The second is the management consideration of the patent examination organization. That a patent application relates to one invention only facilitates the classified management of the patent application (e.g. use of the international patent classification number), and further facilitates the assignment to the examiner of the related technical field for search and examination. Moreover, such classified management also facilitates the public use of patent information, including the consultation of the prior art in the related technical field. However, from the foregoing reasons, the unity principle is only a formal requirement and does not involve a substantive condition of granting a patent right. Therefore, the lack of unity is usually not deemed to be a reason for invalid patent right.

The unity provisions in PCT are described in detail as follows. According to Article 13.1 of the Implementing Rules of the PCT, an international application shall relate to one invention or a group of inventions so linked as to form a single general inventive concept only. According to this article, there are two circumstances under which the unity of invention is required: one invention is involved only; a group of inventions so linked as to form a single general inventive concept is involved only. The former circumstance is a pure embodiment of the unity principle. The latter circumstance can be seen as an extension of the former circumstance, that is, when a group of inventions is linked together as to form a single general inventive concept, the group of inventions can be regarded as "one invention" so that it can comply with the unity principle. Obviously, the latter circumstance is a difficulty in the determination of unity.

Before determining unity, we need to clarify a fundamental question: What is the object of determination of unity? That is, for one patent application, what is the basis of the patent examination organization for determination of unity? Although the unity principle is generally described as "one patent application shall be limited to one invention", it actually means that "one patent application shall only claim one invention". Therefore, the determination of whether a patent application meets the requirement of unity is based on the contents of the claims rather than the description. In theory, in addition to technical contents corresponding to the claims, the description can describe all kinds of contents unrelated to claims such as multiple unrelated inventions. Such descriptions will not be subject to the unity principle, and examiners will at most indicate the preparation defects of the descriptions themselves (e.g. wording and sentence problems) or whether an invention is disclosed sufficiently. However, once an applicant tries to write other inventions recorded in the description into the claims (including replacement of the original claims), the application will be subject to the unity principle.

Back to the determination of unity itself, we need to explain what is "a single general inventive concept"? Only from the literal meaning, it is a relatively broad term and difficult to precisely define the scope. For example, do all technical solutions designed to improve the combustion efficiency of engines belong to a single general inventive concept? In this regard, Article 13.2 of the Implementing Rules of the PCT gives an explanation: Where a group of inventions is claimed in one international application, the requirement of unity of invention referred to in Article 13.1 thereof shall be deemed as met only when there is a technical relationship among those inventions involving one or more same or corresponding special technical features. The expression "special technical features" shall mean those technical features that define a contribution which each of the claimed inventions, considered as a whole, makes over the prior art.

According to the aforesaid provisions, for a group of inventions claimed in one claim, the technical relationship among them is not broad and arbitrary and is deemed to exist only when their technical solutions contain at least one same or corresponding special technical feature. In such case, they are determined to belong to "a single general inventive concept", and thus meet the requirement of unity. Therefore, by introducing the concept of "special technical feature", there is a specific and clear operational standard for determining whether several technical solutions belong to "a single general inventive concept".

The "special technical feature" is a concept designed to determine unity. It shall be understood as the technical feature defining contributions of the inventions to the prior art and giving novelty and creativity of the inventions compared with the prior art, and it shall be determined after each of the claimed inventions is considered as a whole. Thus, unity and inventiveness are linked together through the concept. Here, the main possible reason for emphasis on "each of the claimed invention is considered as a whole" is that: the inventiveness shall be determined based on the whole technical solution defined by the claims, that is, we shall evaluate whether the technical solution, rather than a technical feature, has inventiveness. However, for the determination of unity, we only need to find the technical relationship among different technical solutions and the technical relationship is just embodied on a single technical feature. Therefore, to consider the integral determination of inventiveness and the single determination of unity, the concept of "special technical feature" deliberately points out that when the technical solution of the invention as a whole has inventiveness, a single technical feature that plays a decisive role in term of inventiveness in the technical solution is the special technical feature embodying the technical relationship.

According to the understanding above, if a group of inventions does not involve a same or corresponding technical feature, they may not involve a same or corresponding special technical feature obviously and thus do not have unity. If the same or corresponding technical feature contained is common knowledge in the field, the technical feature may not play a decisive role in the inventiveness of relevant technical solution obviously, and therefore is not a specific technical feature and such inventions clearly do not have unity. In addition to these two distinct circumstances without unity, unity needs to be determined typically after comparison with the prior art and determination of inventiveness. Therefore, unity is often not determined by the receiving office or the international office, but by the international searching authority or the international preliminary examination authority or even by the designated state office.

In the international phase of a PCT application, if an international patent application fails to meet the requirement of unity, generally, the examiner shall indicate such failure and require the applicant to pay fees for the search or examination of additional invention. When an examiner can search the additional inventions by the negligible additional effort and especially when these inventions are very close in terms of ideas, the examiner may decide to search all inventions but should still point out the lack of unity in the application. However, the international preliminary examination authority generally does not perform international preliminary examination on the claims without the international search report. When the examiner points out the problem of unity and requires the applicant to pay a surcharge, the applicant can raise an objection at payment of the surcharge (an objection fee is required to be paid at the same time), and the objection should state that the application complies with the requirement of unity or that the surcharge is too high. In such case, the international searching authority or the national preliminary examination authority shall start the objection procedure and give a ruling on the applicant's objection.

It should be noted that when the international searching authority and/or international preliminary examination authority does not search all the claims, it will not affect validity of this international patent application, and a designated state (or selected state) can still consider all the claims. Of course, the designated state (or selected state) can also require the applicant to pay related fees in accordance with its national laws, otherwise the parts of the international patent application that are not searched and/or examined are deemed to be withdrawn.