[Code of Federal Regulations] [Title 37, Volume 1] [Revised as of July 1, 2002] From the U.S. Government Printing Office via GPO Access [CITE: 37CFR2.91] [Page 287] TITLE 37--PATENTS, TRADEMARKS, AND COPYRIGHTS DEPARTMENT OF COMMERCE PART 2--RULES OF PRACTICE IN TRADEMARK CASES--Table of Contents Sec. 2.91 Declaration of interference. (a) An interference will not be declared between two applications or between an application and a registration except upon petition to the Commissioner. Interferences will be declared by the Commissioner only upon a showing of extraordinary circumstances which would result in a party being unduly prejudiced without an interference. In ordinary circumstances, the availability of an opposition or cancellation proceeding to the party will be deemed to remove any undue prejudice. (b) Registrations and applications to register on the Supplemental Register, registrations under the Act of 1920, and registrations of marks the right to use of which has become incontestable are not subject to interference. [37 FR 2881, Feb. 9, 1972, as amended at 54 FR 34897, Aug. 22, 1989]