By Douglas V. Gibbs
*Article I, Section 9 prohibits the Congress from passing a Bill of Attainder. A Bill of Attainder is when the legislature declares the guilt of a person or group of persons, and punishes them without due process (the benefit of a trial).
In Britain, bills of attainder were used as a convenient way for the King to convict subjects of crimes and confiscate their property without the bother of a trial, and without the need for a conviction or indeed any evidence at all. Such actions were seen as tyrannical because often this power was used against political enemies, and the Founding Fathers did not wish to give the new federal government those same kinds of powers. Some states, prior to the Constitution, did use attainders against British loyalists, but the practice all but disappeared after the Constitution so specifically forbid the use of attainders by the U.S. Congress, and the States.
Prohibiting the use of bills of attainder serves a number of purposes. One purpose is that by disallowing the bills of attainder the separation of powers is reinforced. By disallowing bills of attainder, it literally forbids the legislature from performing a judicial function. Another purpose is in regard to the protection of the concept of due process, which was later reinforced by the Fifth Amendment to the Constitution.
The true danger of a bill of attainder is that such a legislative act inflicts punishment without a judicial trial, and takes away the life, liberty or property of the target [without due process in violation of the 14th Amendment, as well].