The Constitution can be amended (that is, changed) through a formal process established by the Framers in 1787. For an amendment to take effect, it first needs to be officially proposed by a two-thirds vote in both houses of Congress (or by two-thirds of all state legislatures), then it needs to be ratified by three-fourths of the state legislatures. This process is difficult but not impossible to complete; over the course of American history, 27 different amendments to the Constitution have been ratified. Article V includes two restrictions upon amendments, both rooted in the difficult compromises hammered out at the Constitutional Convention to solve controversies over slavery and representation. The first restriction, italicized here because it is no longer operative, barred any amendments that would have outlawed the slave trade before 1808. And the second restriction ensures that no amendment can end the system of equal representation of all states, large and small, in the US Senate.