Final answer:
The 21st Amendment is the only constitutional amendment ratified by approval of state-ratifying conventions rather than the state legislatures. This was a unique occurrence in US constitutional history and was used to repeal the 18th Amendment, which had established Prohibition.
Step-by-step explanation:
The 21st Amendment is the only constitutional amendment that was ratified by approval of state-ratifying conventions. This unique method of ratification stands out in the history of the United States Constitution. The amendment process typically follows the route laid out in Article V of the Constitution, with two primary steps: proposal and ratification. While most amendments have been ratified by three-fourths of state legislatures, the repeal of Prohibition through the 21st Amendment utilized the alternative mode, involving special conventions in each state.
The 21st Amendment's ratification reflected a significant moment in American constitutional history, demonstrating the flexibility of the amendment process and the desire to swiftly reverse the effects of the 18th Amendment, which had instituted Prohibition. This method ensured direct participation by the people and is a testament to the responsive nature of the Constitution to public sentiment. Notably, this method has been employed only this one time throughout the constitutional history of the United States.