**US dollar (before 1944, gold standard)**:

- **Coverage**: Fully backed by gold.

- **Period**: 1792–1944.

- **Special feature**: Dollars could be exchanged for gold.

2. **US dollar (Bretton Woods system, 1944–1971)**:

- **Coverage**: Partially backed by gold (35 USD per ounce).

- **Period**: 1944–1971.

- **Special feature**: Other currencies were tied to the dollar.

3. **US dollar (fiat dollar, from 1971)**:

- **Coverage**: None (based on confidence in the US economy).

- **Period**: 1971–today.

- **Special feature**: Money supply can be flexibly controlled by the central bank.

4. **Petrodollar (from 1973)**:

- **Coverage**: None (part of the fiat system).

- **Period**: 1973–today.

- **Special feature**: Global oil trade is conducted in US dollars.

5. **Digital US dollar (future project)**:

- **Coverage**: None (like fiat dollar).

- **Period**: Planned for the future.

- **Special feature**: Electronic currency, similar to cryptocurrencies.