Causes
A shutdown can happen when a legislative body (including the legislative power of veto by the executive) cannot agree on a budget financing its government programs for a pending fiscal year. In the absence of appropriated funds, the government discontinues providing non-essential services at the beginning of the affected fiscal year. Government employees who provide essential services, often referred to as "essential employees," are required to continue working.
Specifically, in the case of the United States federal government, the Antideficiency Act, together with legal opinions, particularly one written by Attorney General Benjamin Civiletti in 1981, define what is and is not allowed in the absence of an appropriation.
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