Many PCT patentability rules relate to the filing date of the international patent application. For searching and examination purposes, the term "PCT (international) filing date" refers to the relevant date, i.e., the earlier of the actual international filing date or the filing date of a "priority" patent application filed by the applicant(s) containing a disclosure that supports the claimed invention. Under the Paris Convention for the Protection of Industrial Property, a priority application must have been filed no more than 12 months before the actual filing date of the PCT application (PCT Article 4). A priority application may be an international, regional (e.g., EPO) or national patent application. A U.S. national application may be a provisional or regular patent application.
A PCT application may also be filed as a continuation or continuation-in-part application of a copending national patent application. The copending national application must have been filed in a country designated in the PCT application that allows such treatment of its national applications, such as the U.S. In this case, the PCT filing date (with respect to that country only) can be pushed back to the filing date of the earlier-filed national application for the subject matter that was disclosed in that national application.