When selecting a new brand name, some marketers mistakenly believe they cannot use any mark that anyone else uses. That is not correct. It usually is not a requirement that no one else uses the mark, for any product or service. With some exceptions for “famous” marks, a party’s trademark rights generally are limited to the goods or services on which that party uses the mark, as well as products and services that are so similar that consumers would assume a connection between the uses.
So, for instance, two companies probably could coexist in using the mark CREST if one used it for toothpaste and one used it for windbreaker jackets. This is because the respective goods are so different that no one would be likely to assume the uses are connected in some way. The two brands could coexist in the marketplace without any consumer confusion.

"This CHEVROLET brand dental floss is great stuff!"
On the other hand, you would not be well advised to adopt the mark CREST for dental floss, in the face of an existing use of the mark for toothpaste. In this case, the goods are similar enough that consumers are reasonably likely to mistakenly assume a connection between the goods – either that both products are marketed by the same company, or that the well-known CREST toothpaste brand has licensed or otherwise approved the use of CREST on dental floss. The case might be less clear if the uses were somewhat less directly related – a CREST cosmetic dentistry office or a CREST body wash product, for instance.
You should take these concepts into account when considering new brand names. If you have fallen in love with a proposed name, don’t necessarily cross it off your list just because someone else is using the mark on unrelated goods. Instead, consider whether the other party’s goods are similar enough to your own to cause a likelihood of consumer confusion.
At the same time, remember that those enmeshed in an industry may have a distorted view of what goods are related or unrelated. A former colleague of mine once had an opponent in the computer industry assert something to the effect of, “there’s no way consumers would be confused, the products are completely unrelated – yours is a sixteen-pin device and ours is a seventeen-pin device.”
Try to bear in mind that a court probably will consider the issue from the perspective of a consumer far less aware than yourself of the subtle divisions within your industry. Hopefully you have hired a seasoned trademark attorney to help you down the brand selection path – this is a very good time to pay close attention to his or her advice.
Earlier I mentioned special treatment for famous marks. A brand that is “famous” (a legal determination), is given a sort of super-protection under the trademark law, which forbids the use of that mark even for unrelated goods or services. This is to prevent the dilution of the single meaning of the mark in the minds of the public, by “blurring” that association. Dilution is a broad enough topic to deserve a separate discussion at a later date. Suffice it to say, however, that it probably would be a bad idea to choose COCA-COLA as your brand name for any type of goods or services.
PHOTO COURTESY OF FLICKR USER D SHARON PRUITT, UNDER THIS CREATIVE COMMONS LICENSE.
