The US federal government on Tuesday lurched into a partial shutdown, the first in 17 years, as American lawmakers failed to agree to a spending bill for keeping the federal government running beyond midnight Monday.

US fiscal year of 2014 also began Tuesday. Under the Constitution of the United States, the Congress must pass laws for the government to spend money. If Congress can't agree on a spending bill - or if the president vetoes it - the government does not have the legal authority to spend money.

US Congress used to follow a procedure of passing a budget first, then 12 separate appropriations bills. That process has been paralyzed in recent years because of increased political brinkmanship, and the Congress turns to a stopgap continuing resolution, or CR, that maintains federal government spending at existing levels for all or part of the fiscal year.

A shutdown doesn't mean the whole government will come to a halt. Spending for essential functions related to national security and public safety will continue, such as payment for US military troops, while federal employees of non-essential services will face being furloughed to reduce an immediate and critical effect of a government-wide shutdown. There is no assurance of receiving retroactive payment for furlough days once the shutdown crisis is over.

Up to 1 million federal employees of non-essential services would be furloughed because of Tuesday's shutdown, including those in the federal tax agencies, departments of Justice, Labor and the Treasury, and Environmental Protection Agency.

The US Supreme Court would probably operate normally, and the Federal Reserve would also stay open, as it does not depend on congressional appropriations but the interests on the securities to operate.

Government agencies can't ignore the shutdown because under a federal law, it can be a felony to spend taxpayers' money without an appropriation from the Congress. Throughout the shutdown period, agencies will be informed of the latest development regarding resolving of the budgetary impasse.

There have been 17 partial shutdowns of the federal government agencies since 1977, according to the Congressional Research Service. All were caused by funding lapses, and all resulted in furloughing of some federal employees, but most of them were quite brief.

The last major shutdown occurred in late 1995 and early 1996, when Bill Clinton administration and the House of Representatives failed to agree on a budget to fund the government. The shutdown last for 28 days in total, until a new resolution was finally reached.