1. Commodity money
For most of history, money was something scarce: shells, silver, gold. Coins were valued by their metal, but rulers could "debase" them by mixing in cheaper metals.
2. Paper claims on gold
Banks issued notes that could be redeemed for gold. Sweden's Riksbank (1668) and the Bank of England (1694) were among the first central banks.
Redemption was often suspended when it mattered. In 1797 the Bank of England stopped paying notes in gold during war with France (until 1821), and in September 1931 the UK suspended the gold standard after a run on sterling.
3. Bretton Woods (1944–1971)
After World War II, 44 countries tied their currencies to the U.S. dollar, and the dollar was convertible to gold at $35 an ounce for foreign governments. Over time far more dollars existed abroad than the U.S. had gold.
4. Pure fiat (1971–)
On 15 August 1971 President Nixon closed the "gold window". In the Federal Reserve's own historical account: "in effect, the international monetary system turned into a fiat one." Since then, no major currency has been redeemable for anything.
5. Bitcoin (2009–)
Bitcoin's genesis block (3 January 2009) carried a newspaper headline about a bank bailout. Bitcoin returns to a scarce money, but one that is digital, needs no vault and cannot be suspended by any authority.
The pattern
Each time money depended on an issuer's promise, the promise was eventually changed. Bitcoin replaces the promise with rules anyone can verify.