A common-law trademark is a local right, not a national one. It protects the goodwill a business has actually built, in the places it has actually built it, and it can be split between honest competitors who each own the same name in different corners of the country. This guide is the strategy manual for working with that reality rather than being ambushed by it: it maps the three zones of protection, translates each zone into the evidence a court will demand, and shows how the four-factor market-penetration test decides close calls at the edges of a market. It explains how federal registration rewrites the map through constructive notice under 15 U.S.C. § 1072 and constructive use under § 1057(c), why the Dawn Donut rule can still delay an injunction the day after you register, and how e-commerce is quietly dismantling that rule. It closes with concurrent-use proceedings, coexistence drafting, an expansion-and-franchising playbook, a sequential litigation checklist, a worked hypothetical, and an FAQ, all cross-linked to the deeper UpLaw resources on each sub-topic.