This doctrine can be used to extend a trademark's prior rights into a new geographical area, or into a new product line.  The doctrine applies when a company is already using their trademark in one area, and the newly expanded-into area is a "natural" extension of the prior use.  In other words, consumers must be likely to believe that use of the mark in the new area is by the same source using that mark in the old area.

Some courts additionally require a showing that the prior user planned to expand its operations and use of the mark into the newly claimed area prior to the opposing party arriving in that area.