Most sitting American presidents visit Arlington National Cemetery on Memorial Day to lay a wreath on the grave of the Tomb of the Unknowns. By tradition, any presidential visit at this site – or any other U.S. military installation – merits a 21-gun salute. The origins of this tribute date back to the 14th century when battling war ships signaled their defeat by discharging seven cannons, rendering them ineffective. Because gunpowder could be kept in greater quantities on land, forts could fire three rounds for every one from sea so the number of shots rose to 21. In time, the 21-gun salute became the highest honor a nation paid, although throughout the 1800s the number of rounds varied – sometimes according to the number of states in the Union. In 1842, the United States declared the 21-gun salute as its presidential salute. In 1890, the national salute was formally re-established as the 21-gun salute.