According to Article 1, Section 9 of the United States Constitution, Congress is prohibited from passing ex post facto laws (laws making an action a crime after the action has occurred), nor can it pass what is known as a bill of attainder, a law targeting for punishment a specific group of people. Congress is prohibited from suspending habeas corpus except in times of national security concerns, and it is required to publicly account for all money withdrawn from the national treasury. Additionally, Congress is prohibited from taxing inter-state commerce. Congress is given some flexibility through what is sometimes called the "elastic clause" or the "necessary and proper" clause, as it states that Congress can make laws as it see fit as long as these laws are found to be "necessary and proper" to the welfare of the nation. Of course, were this called into question, the President has the power to veto the law, and the Supreme Court could review and uphold, or strike down the law--a demonstration of the checks and balances system. This system that is referred to as "checks and balances" is place for the specific purpose of allowing the President and the Supreme Court to reign in Congress should it step out of line, while Congress has similar powers it can exercise if need be to regulate the other two branches.