Definition
The price at which a bill of exchange drawn in one country upon another country can be bought or sold in the country of origin at a given moment. In practical terms, it is the ratio at which one currency converts into another — the variable market price that determines how much of one nation's money is required to acquire a stated sum of another nation's money.
Rate of exchange governs the real value of international commercial transactions. When a creditor in England holds a draft payable in New York, the rate of exchange at the time of payment determines whether that creditor receives more or less than the face value of the underlying obligation in domestic currency terms. Fluctuation in the rate between the time a contract is made and the time it is performed can create substantial gain or loss, and courts have long been called upon to determine which rate — the rate at contract, the rate at breach, or the rate at judgment — controls the measurement of damages.
Common Language
Modern common usage (Wiktionary): "Exchange rate" — the rate at which one currency can be exchanged for another, typically set by financial markets or central banks.
Historical common usage (Webster's 1913): Not separately defined as a term of art; the concept appeared incidentally in discussions of trade and banking as the price of foreign drafts.
The common and legal meanings point at the same underlying phenomenon, but the legal definition is more specific: it is historically framed around the market price of bills of exchange as instruments, not merely the abstract ratio between currencies. This matters for historical research because pre-twentieth-century legal sources treat the rate of exchange as inseparable from the law of bills and commercial paper, not as a standalone concept in monetary or banking law.
Common Confusion
Rate of exchange is sometimes conflated with par of exchange. Par of exchange is the theoretical equivalency between two currencies based on their respective mint standards — the ideal ratio assuming each currency is of precise legal weight and purity. The rate of exchange, by contrast, is the actual market price at a given time, which may be above par (exchange at a premium) or below par (exchange at a discount). Rapalje & Lawrence draws this distinction explicitly: par describes what currencies are worth in theory; rate of exchange describes what bills actually sell for in practice. The gap between the two is the practical problem that bills of exchange law exists to resolve.
Why It Matters in Research
This term is a gateway to the historical law of commercial paper and international trade. Before modern currency markets, the rate of exchange was determined by the market for bills of exchange, and legal disputes turned on which rate applied at which moment in a transaction's life. Researchers using nineteenth-century sources — treatises, digests, case reporters — will find rate of exchange embedded within discussions of bills of exchange, negotiable instruments, and conflict of laws, not in materials that resemble modern foreign exchange or banking regulation.
Three research traps are common. First, historical sources do not separate the rate of exchange as a monetary concept from the instrument (the bill) through which it was expressed; the two must be researched together. Second, jurisdictional conflict-of-laws rules governing which country's rate applies at which date varied, and courts in England, the United States, and the states often reached different answers. Third, the twentieth-century shift to floating exchange rates and centralized currency markets changed the factual premise underlying older doctrine; cases decided before the Bretton Woods era and cases decided after it may be citing the same rule but operating in fundamentally different commercial environments.
For damages research specifically, the moment-of-breach versus moment-of-judgment question remains a live issue in international commercial arbitration and transnational litigation. Researchers should trace forward from the historical bill-of-exchange context into modern Restatement and uniform law treatments.
Historical Dictionary Support
All four source dictionaries define the term consistently and briefly: the actual price at which a bill drawn in one country upon another may be bought in the former country at a given time. Black's and Burrill cite Story on Bills (§ 31), which is the standard nineteenth-century American treatise authority for the definition. Rapalje & Lawrence adds the most analytical content by distinguishing rate of exchange from par of exchange and gesturing toward the mechanics of mint-based currency valuation. Bouvier's is the most compressed, offering only the market-price formulation without elaboration.
What the historical dictionaries do not address is significant: none discusses how courts determine which rate controls when the rate changes between contract formation and performance, none addresses floating exchange rates (a twentieth-century phenomenon), and none treats the rate of exchange in the context of taxation or domestic transactions — both of which became legally important later. Researchers should treat the historical definitions as accurate but narrow starting points rather than comprehensive statements of the law.
Jurisdictional Note
In the United States, the applicable rate of exchange in litigation — particularly for measuring damages in contracts payable in foreign currency — has been addressed differently by federal and state courts, and the Restatement (Second) of Conflict of Laws addresses the choice of law dimension. International commercial arbitration now applies its own conventions. Researchers should not assume that the historical American common-law rule mirrors the English rule or that either maps onto current practice.
Encyclopedia Cross-Reference
The Law Mind Tax Encyclopedia: Like-Kind Exchanges (tax_149) — relevant where exchange transactions intersect with tax treatment of gains and losses attributable to currency fluctuation.