In the United States, enforceable trademark rights are born the moment a distinctive mark is used in commerce, with no filing, fee, or certificate required. This article explains where those unregistered common law rights come from, why a "first to use" country still lets a later user win in its own backyard, and how the geographic reach of an unregistered mark is fixed by the Tea Rose-Rectanus doctrine drawn from Hanover Star Milling Co. v. Metcalf, 240 U.S. 403 (1916), and United Drug Co. v. Theodore Rectanus Co., 248 U.S. 90 (1918). It shows how courts actually measure a mark's territory through market-penetration and natural-expansion analysis, how a later federal registration freezes a senior common law user in place through constructive use and constructive notice, and how concurrent-use proceedings and the Dawn Donut rule complicate the map. Worked hypotheticals trace priority fights, the good-faith remote-user defense, and proof-of-first-use problems, and the piece closes with guidance on symbols, loss of rights, enforcement under Section 43(a), a practical checklist, and a FAQ, with cross-references throughout the UpLaw library.