In many countries, but not in the United States, which recognizes common law trademark rights, a trademark which is not registered cannot be "infringed" as such, and the trademark owner cannot bring infringement proceedings. Instead, the owner may be able to commence proceedings under the common law for passing off or misrepresentation, or under legislation which prohibits unfair business practices. In some jurisdictions, infringement of trade dress may also be actionable.
Where the respective marks or products or services are not identical, similarity will generally be assessed by reference to whether there is a likelihood of confusion that consumers will believe the products or services originated from the trademark owner.
Likelihood of confusion is not necessarily measured by actual consumer confusion, though normally one of the elements, but by a series of criteria Courts have established. A prime example is the test announced by the Ninth Circuit Court of Appeals in AMF, Inc v Sleekcraft Boats, 599 F.2d 341 (C.A.9) 1979. The Court there announced eight specific elements to measure likelihood of confusion:
Other Courts have fashioned their own tests for likelihood of confusion -- like those announced in In re E.I. du Pont de Nemours & Co., 476 F.2d 1357, 177 USPQ 563 (CCPA 1973), known collectively as the DuPont factors.
If the respective marks and products or services are entirely dissimilar, trademark infringement may still be established if the registered mark is well known pursuant to the Paris Convention. In the United States, a cause of action for use of a mark for such dissimilar services is called trademark dilution.
In some jurisdictions a party other than the owner (eg. a licensee) may be able to pursue trademark infringement proceedings against an infringer if the owner fails to do so.
The party accused of infringement may be able to defeat infringement proceedings if it can establish a valid exception (e.g. comparative advertising) or defence (e.g. laches) to infringement, or attack and cancel the underlying registration (eg. for non-use) upon which the proceedings are based.
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