The most important function of Congress is the legislative function. Article I of the Constitution states that all legislative powers shall be vested in a Congress of the United States, with the House and Senate as equal partners in the legislative process.
The Constitution created a system of checks and balances among the three branches of the federal government. Through its legislative function, Congress exercises authority over the government's financial and budgetary policies and has what is known as the power of the purse. Congress decides tax legislation, pays the country's debts and provides for the defence and welfare of its citizens. Under Article I, Congress has the power to spend and print currency, regulate commerce, establish rules of immigration and naturalization, establish the federal courts and their jurisdictions, establish post offices, issue patents and copyrights, and fix standards of weights and measures.
The legislative function is inextricably intertwined with Congress' authority over budgets and commerce, which make a functioning government possible. Congress also has implied powers derived from the Constitution's necessary and proper clause. Through the necessary and proper clause, Congress has power to carry out its functions, including oversight of the budget prepared by the executive branch and funding the budget with appropriations.