Like many other nations, the United States flies its national flag at half mast or half staff as a sign of mourning and respect for prominent national figures who have passed away such as former Presidents, Supreme Court Justices, Governors, and Members of Congress. Like other activities having to do with the American flag, half masting the flag is governed by the United States Flag Code, which also covers other issues of flag etiquette like how the American flag should be displayed, when it can be flown, and how flags should be handled. Many nations have a similar flag code which is designed to set out rules for the respectful treatment of the national flag.

The United States Flag Code only governs the display of the American flag within the United States. Overseas, the flag should be displayed in accordance with other national flag codes, which often have restrictions on the heights at which foreign flags can be displayed. When flying the flag at half mast, citizens should be aware that the Flag Code designates how the flag should be displayed at half mast, but not when. Federal mandates about half masting the flag only apply to federal agencies, although state agencies and citizens are encouraged to follow suit.

On the federal level, the President of the United States issues orders about when the flag shall be flown at half mast. Common instances when the flag is flown at half mast include the death of a former President, in which case the flag is half masted for 30 days, the death of a Vice President, Chief Justice, or Speaker of the House, when the flag is half masted for 10 days, and the death of other prominent political figures, which is usually accompanied by a single day of flying at half mast. In other instances, the President may issue an order to fly the flag at half mast in the instance of a national disaster, or to commemorate a national day of mourning. The only day of the year in which the American flag is required to fly at half mast is Memorial Day.

On the State level, a state Governor can issue an order to fly the flag at half mast as well, for many of the same reasons that the President does. When the President issues an order to fly the flag at half mast, state Governors usually supplement it with a directive to state agencies. The Governor may also decide to fly the flag at half mast to commemorate the death of a well known figure within the state, or for +a state day of mourning. Individual citizens are welcome to comply with these orders to fly the flag at half mast, and some companies and institutions will half mast their flags if a prominent official such as the Chancellor of a university passes away.

To fly the American flag at half mast, it is first raised all the way to the top of the flag pole and then lowered to the mid point. At the end of the day, the flag is raised to the top of the flagpole before being lowered all the way. On Memorial Day, the flag is only flown at half mast for half of the day: at noon, the flag is raised all the way. Because the American flag within the United States must always fly higher than other flags, flags of states and other nations are usually half masted as well. If two flags are flown on pole, the American flag should as usual be above the other flag.