The custom stems from naval tradition in the sixteenth century, when a warship entering a foreign port would fire each of its cannons while still out of range of targets. Since cannons then required a considerable time to reload, the ship was effectively disarmed, signifying the lack of hostile intent. In the earliest days, seven guns was the recognized British national salute because seven was the standard number of weapons on a vessel. In those days, gunpowder made from sodium nitrate was easier to keep on dry land than at sea. The early regulations stated that although a ship would fire only seven guns, the forts ashore would fire three shots to each one shot afloat, hence the number 21.