If, after your search, you determine that the name you've chosen (or a similar name) does not already belong to someone else, you can go ahead and use it.
On the other hand, if your search turns up an identical or similar name to the one you want to use, you may or may not be able to use it, depending on the circumstances.
As a first rule, if your desired name uses part of a well-known or heavily marketed trademark, pick a new one right away. Don't risk the expense of a possible fight with a big corporation, not to mention the costs of changing the name on all of your business materials.
Also, if your desired name uses part of a name that's already registered for official trademark protection, especially at the federal level, you should take that as a "No Trespassing" sign and pick another name -- even if the name isn't very well known. Owners of federally registered trademarks have the right to use their trademarks anywhere in the country, and it is easy for them to bring and win lawsuits against trademark violators.
There are a few instances when taking a name that's already in use is okay (as long as the name isn't famous). If the name is being used by a company that provides a very different product or service than the one you plan to sell, then you can probably move forward with your plans to use the name. This is especially true if the two businesses serve only local markets and are hundreds of miles apart.
The key here is whether your use of the name, or something similar, would confuse customers about the origin of the product or service. For example, just because a plumbing business in Coos, Oregon, calls itself Z-Pop doesn't mean you, in Arizona, can't use Z-Pop as the brand name for your soda pop. That plumbing business in Oregon is not your competitor and your use of Z-Pop for soda pop will not likely confuse customers into thinking that your soda pop is related to the plumbing business. On the other hand, if Z-Pop is being used to market another soda pop product, you should choose a different name.
For more information on conflicting trademarks and customer confusion, see Nolo's Trademark Law area.
For help in choosing and protecting the name for your business, including the latest on trademark searches and registrations, get Trademark: Legal Care for Your Business and Product Name, by Stephen R. Elias (Nolo).
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