Primary v. Supplemental Registrations
The USPTO maintains a primary (or principal) register for strong or distinctive trademarks and a secondary (or supplemental) register for weak trademarks. Most TESS Search results display registered trademarks from both the primary AND the supplemental registers, and, in both cases, relevant principal and supplemental registrations can both be cited against your application by the USPTO. Further, both types of registrations enable your competitors to sue you in federal court if they believe you have infringed them.
However, because supplemental registrations have been placed on the secondary register for being weak, finding a competitor’s supplemental trademark tells you immediately that their ability to enforce it against you is an uphill battle that may actually be worth your risk in using the mark.
It also means that, assuming you can clear the review of the examiner, your competitor will have no right to use their mark to oppose your application, though they may attempt to use it to cancel your trademark after it registers. If your trademark search reveals the existence of supplemental registrations you should consult an experienced attorney, such as those at the Trademark Law Firm, before proceeding.
Principle registrations, in contrast, are much more serious because they carry the presumption that they are strong, valid, and enforceable everywhere that the United States has laws. If your trademark search reveals a strong similarity between your mark and a competitor’s principle registration and you proceed regardless you may very likely find yourself fighting a two-front battle against the USPTO Examiner AND your competitor. Further, if you use or apply for a trademark that has been on the principle register long enough to become incontestable, you may even find the USPTO and Courts siding with your competitors without even hearing your defense.
By performing a trademark search prior to applying to register your own trademark you will have all the information you need to avoid the problems above, or at the very least proceed with a full knowledge of your risks and liabilities.