What do the kind codes ‘C1’, ‘C2’, and ‘C3’ represent in U.S. patent documents?

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.

The kind codes ‘C1’, ‘C2’, and ‘C3’ are used for specific types of U.S. patent documents. According to MPEP 901.04(a):

• “C1” – Reexamination certificate issued after the first reexamination proceeding
• “C2” – Reexamination certificate issued after the second reexamination proceeding
• “C3” – Reexamination certificate issued after the third reexamination proceeding

These kind codes are important for identifying reexamined patents and their subsequent certificates. Reexamination is a process where the validity of a granted patent is reconsidered based on prior art. The ‘C’ series kind codes help patent examiners and researchers quickly recognize that a patent has undergone one or more reexamination procedures and that changes may have been made to the original patent claims.