A trademark is a word, phrase, symbol or design, or a combination thereof, that identifies and distinguishes the source of the goods of one party from those of others. Trademarks, unlike patents, can be renewed forever as long as they are being used in business.
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Registered Trademarks refer to trademarks that have been registered by the United States Patent and Trademark Office (USPTO). Only registered trademarks are permitted to use the r symbol.
State Trademarks as the name suggests, are trademarks that have been registered by a state, not the USPTO. These trademarks are only valid in the state where they are registered. State trademarks use the TM symbol. The Secretary of State is the entity in charge of trademarks for South Carolina.
Findlaw maintains a site with trademark information from all 50 states.
Common Law Trademarks are trademarks that have not been registered by the USPTO, nor have they been registered in any state. It is not necessary to register a trademark. You can establish legal rights simply by using the trademark in commerce. Common law trademarks use the TM symbol. State courts have jurisdiction for these trademarks.
Trade Secrets are information that companies keep secret to give them an advantage over their competitors. The formula for Coca-Cola is the most famous trade secret. Trade secrets are protected by state laws, not trademarks or copyrights.
Resources available at the USPTO website: