How did the Twenty-One gun salute originate?
Here is the official explanation of the twenty-one gun salute, from the chief of the news branch of the U.S. Army Public Information Division:
The firing of gun salutes dates back to the early days of the British Navy. At that time, guns could not be loaded quickly so the act of firing one in a salute indicated that the saluter had disarmed himself in deference to the person being saluted. Since twenty-one guns was the number found on one side of one of the larger "ships of the line", firing all of them became the highest mark of respect, reserved for heads of the state. Smaller numbers of guns were fired in salutes to people of lesser importance. But for all salutes, only odd numbers are used, reflecting the old seagoing superstition against even numbers. This form of saluting was first recognized in the United States in 1875. As Commander-in-chief, the President is accorded with the highest salute of twenty-one guns.