What is a “common law” trademark?

Mere use of a trademark in commerce is enough to create a common law trademark. Common law trademarks provide little protection. The owner of the common law trademark should use the TM symbol to indicate owner’s intent to protect his or her trademark rights in a particular mark. Please note that while the common law does afford the trademark owner some minimum level of trademark protection it should not be considered a valid alternative to the federally registered trademark which offers the greatest protection.