This article analyzes the stability of bimetallism for countries operating in integrated bullion markets that enact different legal ratios. I articulate a new theoretical framework to demonstrate that two countries can both be bimetallic only if they coordinate their legal ratios. The theoretical framework is applied to the mid-eighteenth century when London's legal ratio was 3.8 percent higher than that of Amsterdam. I find that Amsterdam was effectively on the bimetallic standard, whereas London was on a de facto gold standard.