37 C.F.R. § 2.80 Publication for opposition.
Title 37 - Patents, Trademarks, and Copyrights
If, on examination or reexamination of an application for registration on the Principal Register, it appears that the applicant is entitled to have his mark registered, the mark will be published in the Official Gazette for opposition. The mark will also be published in the case of an application to be placed in interference or concurrent use proceedings, if otherwise registrable. [41 FR 758, Jan. 5, 1976]
Title 37: Patents, Trademarks, and Copyrights
PART 2—RULES OF PRACTICE IN TRADEMARK CASES
Publication and Post Publication
§ 2.80 Publication for opposition.

