During the debates on the adoption of the Constitution, its opponents repeatedly charged that the Constitution as drafted would open the way to tyranny by the central government. Fresh in their minds was the memory of the British violation of civil rights before and during the Revolution. They demanded a "bill of rights" that would spell out the immunities of individual citizens. Several state conventions in their formal ratification of the Constitution asked for such amendments; others ratified the Constitution with the understanding that the amendments would be offered.
On September 25, 1789, the First Congress of the United States therefore proposed to the state legislatures 12 amendments to the Constitution that met arguments most frequently advanced against it. Articles 3 to 12, ratified December 15, 1791, by three-fourths of the state legislatures, constitute the first 10 amendments of the Constitution, known as the Bill of Rights. Article 2 concerning “varying the compensation for the services of the Senators and Representatives” was finally ratified on May 7, 1992 as the 27th Amendment to the Constitution. The first amendment, which concerned the number of constituents for each Representative, was never ratified.
For more history and background on the Constitution and the Bill of Rights, read A More Perfect Union: The Creation of the U.S. Constitution at the National Archives' new Charters of Freedom site.