21 Gun Salute

The origin of gun salutes is usually attributed to soldiers or other armed types demonstrated peaceful intentions by placing their weapons in a position that rendered them ineffective. As cannons and small arms came into use, a good way to render them ineffective and demonstrating peaceful intentions, was to fire them, as reloading was a real pain. At sea, seven shots became the norm, probably because of superstition and mysticism about the number seven. On land, with a less limited supply of gunpowder, they could fire three guns for every one shot from a ship, so a salute from a ship of seven guns would be answered by a salute from the shore batteries of 21 guns. When gunpowder technology and storage improved, ships at sea adopted the salute of 21 guns.
There is a complex protocol for salutes. Twenty one guns are only used to salute a national flag, the sovereign or chief of state of a foreign nation, a member of a reigning royal family, and anyone who has ever been elected President of the US. A vice-president, speaker of the house, American or foreign ambassador, a premier or prime minister (unless sovereign), chief justice, cabinet member, state governor, secretary or ranking general of a branch of the armed forces, and president pro tem of the senate all receive 19 gun salutes on entering. Generals, admirals, the assistant secretary of defense, and chairpersons of House committees receive 17. There are 15, 13, and 11 gun salutes for people of descending rank, both military and civilian.

For a full-honor funeral at Arlington, a President gets 21 guns. A secretary of defense, chairman of the Joint Chiefs of Staff, or other military officer given command over multiple branches of the service receives 19. Seventeen guns are fired for a four-star general, 15 for a three-star, 13 for a two-star, and 11 for a one-star.