How is priority of invention determined in interference proceedings?

This page is an FAQ based on guidance from the Manual of Patent Examining Procedure. It is provided as guidance, with links to the ground truth sources. This is information only: it is not legal advice.

Priority of invention in interference proceedings is determined based on several factors, including conception, reduction to practice, and diligence. The MPEP provides priority time charts to illustrate various scenarios. Generally:

• The first to conceive and reduce the invention to practice is awarded priority.
• If one party conceives first but reduces to practice second, they can still win if they show diligence from just before the other party’s conception date until their own reduction to practice.

As stated in the MPEP:

“The first of many to reduce an invention to practice around the same time will be the sole party to obtain a patent in some instances, […] unless another was the first to conceive and couple a later-in-time reduction to practice with diligence from a time just prior to when the second conceiver entered the field to the first conceiver’s reduction to practice.”