A promise made concrete: this much paper always converts to this much metal. Break that promise, and you’ve left the gold standard, whatever you still call the currency.
For most of the century after 1870, the major economies operated on some form of gold standard — currency values fixed to a specific weight of gold, redeemable on demand. The postwar Bretton Woods system (1944) rebuilt this after the interwar collapse: the US dollar was pegged to gold, and every other major currency was pegged to the dollar, giving the world a single, gold-anchored reserve currency. On 15 August 1971, Richard Nixon closed the “gold window,” ending dollar-to-gold convertibility and, with it, the last formal link between money and a physical, unexpandable asset.
What a gold peg actually does is remove a lever: a government on a gold standard cannot expand its money supply beyond its gold reserves without either acquiring more gold or breaking the peg outright. That is precisely why it was abandoned — not because it stopped “working,” but because it stopped permitting the discretionary monetary expansion a fiat system allows. Whether that expansion has served the public or the institutions closest to the money-creation process is the live argument gold-standard critics and defenders are actually having.
Source: Bordo, M.D., “The Bretton Woods International Monetary System: An Historical Overview,” NBER Working Paper 4033, 1992; Gallarotti, G.M., “The Classical Gold Standard as an International Monetary Regime,” in The Anatomy of an International Monetary Regime, Oxford University Press, 1995.