Constructive use under 15 U.S.C. § 1057(c) is the doctrine that turns a trademark application's filing date into a nationwide priority date, treating the applicant as if it had used the mark everywhere in the country the day it filed. This guide reads the statute clause by clause and explains why the priority is contingent on registration, why it relates back to the filing date, and how it differs from the constructive notice of § 1072. It maps the three "except for" carve-outs that protect prior users, prior applicants, and foreign-priority claimants, and it works through how the doctrine interacts with the Tea Rose-Rectanus common-law framework and the § 1115(b)(5) limited-area defense. Along the way it analyzes Zirco, WarnerVision, Kelly Services, Sengoku, Allard, Dorpan, and Dawn Donut, the burden-shifting structure that decides most territorial fights, and the § 44(d) and Madrid § 66(a) routes that can push an effective date behind any visible U.S. filing. It closes with invented worked examples, edge cases, a litigation-and-prosecution playbook, and an FAQ.