Flags are flown from ships to indicate the country or steamship line to which the vessel be-longs. The national flag flown from a ship is called an ensign. Warships which have a fleet commander aboard fly the commander's flag, and are known as flagships. An interesting naval custom is sometimes followed by a naval ship entering a foreign port. The ship will raise the ensign of the foreign nation and fire a cannon salute.

When a ship or military post surrenders to an enemy, it hauls down, or strikes, the national flag. A ship salutes a passing vessel by lowering, or dipping, its national ensign slightly. A flag raised only halfway up a staff is said to be at half-staff (on land) or half-mast (at sea). Flags are flown at half-staff as a tribute to the dead, especially when great national leaders die. A flag which is flown upside down or reversed is a signal of distress. Colors for marching troops and standards for mounted troops are carried on flagstaffs at the front of the formation. The flags are square-ended and usually fringed. Individual companies, troops, and batteries are identified by small flags called guidons, which are used by the troops for guide purposes, to help maintain straight lines and formations.

On the battlefield a white flag, called a flag of truce, indicates either sur-render or a wish to discuss peace terms. The "cease firing" order is usually given when an enemy soldier appears holding up a white flag or handkerchief. The international code "0" flag is displayed over a ship under quarantine. It indicates a contagious disease on the ship and warns people to stay away. Red flags have often been used by extreme revolutionary groups, such as the Commune of Paris in 1871. A black flag decorated with a skull and crossbones is the traditional emblem of pirates.