In Tijuana, Mexico, across the border from San Diego, California, dollars and pesos, English and Spanish, US and Mexican commodities circulate apace. Moving beyond both the old fascination with transnational flows and the emphasis on enforcement and prohibition in current research on international borders, this article examines the everyday pragmatics involved in engaging these disparate forms. In multiple contexts and for varied reasons, actors draw them together as sets of commensurables, attempting to claim equivalence between two national regimes of value and thus consolidate their own standing with respect to a range of interlocutors. But even as they do so, their forceful assertions of commensurability feather apart in the face of a persistent remainder which they themselves evoke: the excess value that may attach to US forms, a qualitative difference that seems to fly in the face of comparability. As this inequality emerges in moments of circulation (display, exchange, ascription of possession to others, and so on), it disrupts even the most quotidian attempts at arithmetic conversion, literal translation, or the seemingly straightforward practicalities of purchase. Not all, however, are equally positioned to reap the interactive benefits of either commensuration or the sense of disproportion that interrupts it. By tracking how different subjects move between those two possibilities, the article opens a novel perspective on the complex interweaving of social difference across the border and within Mexico.