Article Five of the United States Constitution
At first, when the Founding Fathers were writing the Constitution, they were only going to give Congress the power to propose amendments.  However, some of the Founders, like Alexander Hamilton and James Madison, worried that this gave Congress too much power.  Madison and Hamilton re-wrote Article Five to add the second option for proposing an amendment, and the rest of the Founders agreed.  Adding the second option helped make sure the states had power too  the power to propose amendments if enough of their legislatures agreed.