Reflections on Milton Friedman's contributions to open economy money/macro

Friedman's central contributions to open economy macroeconomics are contained in his essay “The Case for Flexible Exchange Rates.” The paper describes equilibrium in an open economy in terms of the flow market for domestic currency, and so presents the basic elements of The Monetary Approach to the Balance of Payments, which Mundell and Johnson developed later in the decade. An application of the argument includes the first definitive statement of the criteria which one finds in the literature on Optimum Currency Areas. The essay's model, in a better specified, more modern form, reappears in A Monetary History (with Anna Schwartz), where it serves as a basis for detailed analysis of particular episodes and as a framework for empirical work which continues to this day.