A brief history of the 21-gun salute

On Memorial Day, the U.S. honors those who've died in service with a 21-gun salute. But where did the practice come from?

Saluting started out as a way to ritually disarm a weapon, signifying peaceful intentions. Ships coming into harbor would often first discharge their cannons to signify they were unloaded.

Initially, U.S. gun batteries would salute by firing one shot for each state in the Union.

The practice of firing 21 shots in salute was formally adopted by the U.S. in 1875 to match the international salute. Now it's used in ceremonies honoring the national flag, visiting dignitaries and the president.

A 21-gun salute differs from the three-volley salute typically seen at military funerals. That practice stems from a 17th-century European cease-fire tradition. After both sides of a battle had paused to tend to their dead and wounded, three rounds would be fired to start the fighting again.

This video includes clips and images from the U.S. Department of Defense.

See images from Memorial Day:

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Soldiers participate in 'Flags In' tradition for Memorial Day at Arlington National Cemetery
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Soldiers participate in 'Flags In' tradition for Memorial Day at Arlington National Cemetery
Members of the 3rd Infantry Regiment (Old Guard) take part in "Flags-in", where they will place a flag at each of the 284,000 headstones at Arlington National Cemetery ahead of Memorial Day in Arlington, Virginia, U.S., May 25, 2017. REUTERS/Kevin Lamarque
A soldier from the 3rd U.S. Infantry Regiment (Old Guard) takes part in "Flags-in", where a flag is placed at each of the 284,000 headstones at Arlington National Cemetery ahead of Memorial Day, in Arlington, Virginia, U.S., May 25, 2017. REUTERS/Kevin Lamarque TPX IMAGES OF THE DAY
A member of the US Army places an American flags on a grave at Arlington National Cemetery on May 25, 2017 in Arlington, Virginia in preparation for Memorial Day. / AFP PHOTO / Brendan SMIALOWSKI (Photo credit should read BRENDAN SMIALOWSKI/AFP/Getty Images)
A soldier from the 3rd U.S. Infantry Regiment (Old Guard) holds a handful of flags during "Flags-in," where a flag is placed at each of the 284,000 headstones at Arlington National Cemetery ahead of Memorial Day, in Arlington, Virginia, U.S., May 25, 2017. REUTERS/Kevin Lamarque
Soldiers from the 3rd U.S. Infantry Regiment (Old Guard) take part in "Flags-in", where a flag is placed at each of the 284,000 headstones at Arlington National Cemetery ahead of Memorial Day, in Arlington, Virginia, U.S., May 25, 2017. REUTERS/Kevin Lamarque
A soldier from the 3rd U.S. Infantry Regiment (Old Guard) takes part in "Flags-in", where a flag is placed at each of the 284,000 headstones at Arlington National Cemetery ahead of Memorial Day, in Arlington, Virginia, U.S., May 25, 2017. REUTERS/Kevin Lamarque
Soldiers in the Old Guard walk to place flags at graves in Arlington National Cemetery during 'Flags In' in preparation for Memorial Day May 25, 2017 in Arlington, Virginia. / AFP PHOTO / Brendan Smialowski (Photo credit should read BRENDAN SMIALOWSKI/AFP/Getty Images)
The Chief of Staff of the U.S. Army General Mark Milley (L) and his wife Hollyanne (R) speak with family of a soldier at Arlington National Cemetery as former U.S. Navy corpsman Donna Russo of Boston rests upon the headstone of Jason Allen Lehto during the the "Flags-in" event at Arlington National Cemetery ahead of Memorial Day in Arlington, Virginia, U.S., May 25, 2017. Russo said she was with Lehto when he was killed in Iraq. REUTERS/Kevin Lamarque
Soldiers from the 3rd U.S. Infantry Regiment (Old Guard) salute as Taps is heard nearby during "Flags-in", where a flag is placed at each of the 284,000 headstones at Arlington National Cemetery, ahead of Memorial Day, in Arlington, Virginia, U.S., May 25, 2017. REUTERS/Kevin Lamarque
A soldier from the 3rd U.S. Infantry Regiment (Old Guard) takes part in "Flags-in", where a flag is placed at each of the 284,000 headstones at Arlington National Cemetery, ahead of Memorial Day in Arlington, Virginia, U.S., May 25, 2017. REUTERS/Kevin Lamarque
A soldier from the 3rd U.S. Infantry Regiment (Old Guard) takes part in "Flags-in", where a flag is placed at each of the 284,000 headstones at Arlington National Cemetery, ahead of Memorial Day, in Arlington, Virginia, U.S., May 25, 2017. REUTERS/Kevin Lamarque
Soldiers in the Old Guard arrive to place flags at graves in Arlington National Cemetery during 'Flags In' in preparation for Memorial Day May 25, 2017 in Arlington, Virginia. / AFP PHOTO / Brendan Smialowski (Photo credit should read BRENDAN SMIALOWSKI/AFP/Getty Images)
A soldier from the 3rd U.S. Infantry Regiment (Old Guard) takes part in "Flags-in", where a flag is placed at each of the 284,000 headstones at Arlington National Cemetery, ahead of Memorial Day, in Arlington, Virginia, U.S., May 25, 2017. REUTERS/Kevin Lamarque
ARLINGTON, VA - MAY 25: Sgt. Iwona Kosmaczewska assists with the process of placing flags at the headstones of U.S. military personnel buried at Arlington National Cemetery in preparation for Memorial Day May 25, 2017 in Arlington, Virginia. 'Flags-In' has become an annual ceremony since the 3rd U.S. Infantry Regiment (The Old Guard) was designated to be an Army's official ceremonial unit in 1948� (Photo by Win McNamee/Getty Images)
Soldiers in the Old Guard walk over a puddle while placing flags at graves in Arlington National Cemetery during 'Flags In' in preparation for Memorial Day May 25, 2017 in Arlington, Virginia. / AFP PHOTO / Brendan Smialowski (Photo credit should read BRENDAN SMIALOWSKI/AFP/Getty Images)
Soldiers in the Old Guard wait to place flags at graves in Arlington National Cemetery during 'Flags In' in preparation for Memorial Day May 25, 2017 in Arlington, Virginia. / AFP PHOTO / Brendan Smialowski (Photo credit should read BRENDAN SMIALOWSKI/AFP/Getty Images)
ARLINGTON, VA - MAY 25: Sgt. Iwona Kosmaczewska (L) and Pvt. 2 Wesley Defee (R), members of the 3rd U.S. Infantry Regiment, place flags at the headstones of U.S. military personnel buried at Arlington National Cemetery, in preparation for Memorial Day May 25, 2017 in Arlington, Virginia. 'Flags-In' has become an annual ceremony since the 3rd U.S. Infantry Regiment (The Old Guard) was designated to be an Army's official ceremonial unit in 1948� (Photo by Win McNamee/Getty Images)
Soldiers in the Old Guard prepare to place flags at graves in Arlington National Cemetery during 'Flags In' in preparation for Memorial Day May 25, 2017 in Arlington, Virginia. / AFP PHOTO / Brendan Smialowski (Photo credit should read BRENDAN SMIALOWSKI/AFP/Getty Images)
ARLINGTON, VA - MAY 25: Members of the 3rd U.S. Infantry Regiment place flags at the headstones of U.S. military personnel buried at Arlington National Cemetery, in preparation for Memorial Day May 25, 2017 in Arlington, Virginia. 'Flags-In' has become an annual ceremony since the 3rd U.S. Infantry Regiment (The Old Guard) was designated to be an Army's official ceremonial unit in 1948� (Photo by Win McNamee/Getty Images)
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