The General Interpretive Rules (GIRs) are the legal backbone of the Harmonized System. In the intervening years since the Harmonized System was first unveiled in 1983, a substantial and ever-growing body of articles, commentaries, administrative binding rulings, and, most important, judicial opinions has been published about the scope and application of the GIRs – and yet a conspicuous gap exists in the literature with regard to one particular rule: GIR 4. This article attempts to fill that gap by exploring the intent, circumstances, barriers, and practices that govern the legal application of this rarely used rule. Constrained by the scarcity of insightful authoritative guidance on GIR 4, this article necessarily is premised upon a plain-meaning construction of the rule and to a significant extent on inference drawn from the author’s practical experience with the Harmonized System.