Under the Patent Cooperation Treaty (PCT), an international application, which is also called PCT application, "shall relate to one invention only or to a group of inventions so linked as to form a single general inventive concept". If the requirement of unity of invention is not met, the International Searching Authority (i.e., the patent office in charge of carrying out the international search) "is entitled to request the applicant to pay an additional search fee for each invention beyond the first which is to be searched".