Prior to the passage and implementation of the Leahy-Smith America Invents Act (AIA), the United States patent system was based on the concept of "first to invent".  A "first to invent" patent system credits the inventor who first created an invention.  In this system if two patent applications are filed claiming the same invention, the USPTO would review the evidence of conception, reduction to practice, and diligence to determine which patent application would move forward based on the date of "invention".  Congress passed the Leahy-Smith AIA in 2011 and in 2013 the United States enacted the "first to file" patent system whereby the effective filing date of a patent application with the US Patent and Trademark Office determines priority.