Article 1, Section 8, Clause 5: Currency, weights, and measures

To coin Money, regulate the Value thereof, and of foreign Coin, and fix the Standard of Weights and Measures;

This section enumerates specific powers in connection with the general power of the Congress to regulate interstate and foreign commerce. Since such commerce is dependent on a stable currency, Congress is given power to coin money and regulate its value, including with respect to foreign currency. Congress is also given power to establish a uniform national system of weights and measures.

These powers seem straightforward, but there are some interesting historical wrinkles. The Congress has not established a single system of weights and measures, but multiple systems in parallel. American leaders were interested in the metric system even before the customary system of units was formally regulated in 1833, and metric units were formally established as a parallel system of units in 1866 and have remained so since.  However, various efforts to establish the metric system as the official unit system of the United States have not come to fruition.

It has been argued that the power to coin money is distinct from the power to print paper currency or to establish a fiat currency. However, the next clause mentions the securities of the Unites States, and, historically, paper currency (also known as bank notes) have long been recognized as a form of security. It seems to us that the issuance of bank notes is within the powers granted Congress by this clause.  In practice, fiat currency has shown a dangerous potential to be manipulated, but it has also mostly ensured an adequate money supply in times of rapid economic growth.