"Common Law Trademarks" are a lower level of Intellectual Property protection than filed and approved trade marks. A still higher standard is "Registered Trademarks." Common Law Trademarks are to Registered Trademarks, what Common Law Marriage is to Legal Marriage. It has limited rights, and can be subordinated to a higher level of protection.
"Common Law Trademark" is to simply put a superscript "TM" after any word or phrase and presto, that is a common law trademark. If someone else can show they were using before you in TRADE, then you lose.
"Trademark Pending" means you have paid the fees and submitted a formal request to the US Patent and Trademark Office, and it is in process. This takes precedence over any Common Law Trademark.
"Registered Trademark" means you can legally replace the superscript TM with a superscript R. This means the trademark has been assigned legally, and is as good as it gets. However, if challenged, it is still up to the owner of the Registered Trademark to bring a lawsuit and pay for enforcement.
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