U.S. patent law
The United States previously used a first-to-invent patent system, unlike most other countries in the world. Invention in the United States is generally defined to comprise two steps: (1) conception of the invention and (2) reduction to practice of the invention. When an inventor conceives of an invention and diligently reduces the invention to practice (by filing a patent application, by practicing the invention, etc.), the inventor's date of invention will be the date of conception. Thus, provided that an inventor is diligent in actually reducing an application to practice, he or she will be the first inventor and the inventor entitled to a patent, even if another files a patent application, constructively reducing the invention to practice, before the inventor.
However, the first applicant to file has the prima facie right to the grant of a patent. Should a second patent application be filed for the same invention, the second applicant can institute interference proceedings to determine who was the first inventor (as discussed in the preceding paragraph) and thereby who is entitled to the grant of a patent.
However, the America Invents Act, signed by Barack Obama on September 16, 2011, switched the U.S. patent system from a "first-to-invent" system to a "first-inventor-to-file" system for patent applications filed on or after March 16, 2013.