The priority date is the earliest date to which a patent application can claim priority. The priority date defines what materials are prior art to the patent application; thus, obtaining the earliest possible priority date can be important to the patentability of some inventions. In some cases, the filing date sets the priority date. However, for a utility patent application based on a previous provisional patent application, the priority date may be the filing date of the provisional patent application. Similarly, in the case of continuation patent applications, the priority date is often earlier than the filing date, it is the earliest filing date for any of the parent applications. Also, in PCT patent applications filed after provisional or utility patent applications, the priority date is the earliest filing date of the related applications.