Article I Section 9 of the United States Constitution prohibits Congress from six specific areas of legislation. However, the first limit placed on congressional power was a limit on regulating the slave trade which did not extend beyond the year 1808. Article I Section 9 also prohibited Congress from imposing direct taxes, but this was overturned in 1913 with the adoption of the 16th Amendment. Today, there are four remaining relevant powers denied to Congress in the U.S. Constitution.

The Writ of Habeas Corpus

The second and third clauses of Article I Section 9 limit congressional powers in ways designed specifically to protect the rights of citizens accused of crimes. The second clause prohibits Congress from suspending the Privilege of the Writ of Habeas Corpus. Habeas Corpus, literally meaning "produce the body," grants prisoners the right to appear in a court of law in order to challenge their imprisonment.

Bills of Attainder and Ex Post Facto Laws

The third clause prohibits Congress from passing bills of attainder and ex post facto laws. Bills of attainder are laws directed at specific people or groups of people. Because of this clause, only the courts may determine an individual's guilt. Ex post facto laws are laws that make an action illegal after that action has been committed. By prohibiting ex post facto laws, the framers ensured that all citizens would have the ability to know an action was illegal before committing it.

Export Taxes

The fifth clause reads very simply, "No Tax or Duty shall be laid on Articles exported from any State." This means that Congress is prohibited from imposing export taxes on goods transported across state lines and also across international borders. This clause was written to prevent Congress from discriminating against any particular state or region of the country.

Port Preference Clause

The sixth clause, also called the Port Preference Clause, was also written with the aim of preventing Congress from discriminating against certain states or regions. This clause prohibits Congress from passing any commercial or trade regulations that favor the ports of one state over the ports of another state. Under this clause, ships bound from one port to another cannot be required to stop and pay duties in a third location.