You have a trademark in your trade name, and you have priority in "your" geographical area. A geographic area can be as small as a few square miles, and as broad as the entire nation. You need to consult with a business lawyer about your rights. Avvo has several goods lawyers from the Sacramento area that can help.
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As my colleague states you may each have common law trademark rights which extend to a geographic region. However, based on the actual brand in question here and on the nature of the business, it may be ill-advised to permit anyone, former business partner or not, to use the same or confusingly similar name to promote the same service as it may dilute or nullify your trademark. The best strategy here is to negotiate ownership of the trademark vis a vis your ex-partner if this term was not previously negotiated during the split and consider trademark registration as it may prove to be critical.
If you wish to further discuss trademark protection and enforcement strategy, feel free to contact my office.
L.A. Tech & Media Law Firm
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Because trademark disputes are highly fact-dependent, I agree with my colleagues: If you want a definitive answer, you will need to retain a business lawyer who is familiar with trademark law.
This information does not constitute legal advice and does not establish an attorney-client relationship.