Unregistered trademarks may be protected under common law rights, which are based on usage. Common law marks do not enjoy the same level of protection as registered trademarks. However, they can create problems and complications for businesses that wish to use a similar mark. Trademark ownership is based on who uses a mark first, so it is a good idea to search common law marks in order to avoid future conflicts and problems.
Below are useful resources and suggestions for searching common law marks.
It is possible to register a state trademark in all fifty states. Although state trademarks are relatively weak compared to federal trademarks, they can cause unexpected problems. If you intend to do business in one or more states, it's a good idea to include state trademarks in your clearance search.