Origin of the 21-Gun Salute

Because someone asked, and my guess that "I think it's probably because three and seven are both mystical/significant/lucky numbers" didn't satisfy my curiosity¹...

From the U.S. Army Center of Military History

The tradition of rendering a salute by cannon originated in the 14th century as firearms and cannons came into use. Since these early devices contained only one projectile, discharging them once rendered them ineffective. Originally warships fired seven-gun salutes--the number seven probably selected because of its astrological and Biblical significance. Seven planets had been identified and the phases of the moon changed every seven days. The Bible states that God rested on the seventh day after Creation, that every seventh year was sabbatical and that the seven times seventh year ushered in the Jubilee year.

Land batteries, having a greater supply of gunpowder, were able to fire three guns for every shot fired afloat, hence the salute by shore batteries was 21 guns. The multiple of three probably was chosen because of the mystical significance of the number three in many ancient civilizations. Early gunpowder, composed mainly of sodium nitrate, spoiled easily at sea, but could be kept cooler and drier in land magazines. When potassium nitrate improved the quality of gunpowder, ships at sea adopted the salute of 21 guns.

The 21-gun salute became the highest honor a nation rendered. Varying customs among the maritime powers led to confusion in saluting and return of salutes. Great Britain, the world's preeminent seapower in the 18th and 19th centuries, compelled weaker nations to salute first, and for a time monarchies received more guns than did republics. Eventually, by agreement, the international salute was established at 21 guns, although the United States did not agree on this procedure until August 1875.

¹ - And because I didn't have anything else ready to write about, nor the time to find something and do it, this morning...