House flag

A maritime flag is a flag designated for use on ships, boats, and other watercraft. Naval flags are considered important at sea and the rules and regulations for the flying of flags are strictly enforced. The flag flown is related to the country of registration: so much so that the word "flag" is often used symbolically as a synonym for "country of registration".

Ensigns are usually required to be flown when entering and leaving harbour, when sailing through foreign waters, and when the ship is signalled to do so by a warship. Warships usually fly their ensigns between the morning colours ceremony and sunset when moored or at anchor, at all times when underway, and at all times when engaged in battle—the "battle ensign". When engaged in battle a warship often flies multiple battle ensigns. This tradition dates from the era of sailing vessels. Tradition dictated that if a ship lowered its ensign it was deemed to have surrendered. Masts were targets of gunfire, and the second and subsequent ensigns were flown in order to keep the ensign flying even after a mast hit.

Jacks are additional national flags flown by warships (and certain other vessels) at the head of the ship. These are usually flown while not underway and when the ship is dressed on special occasions. Jacks in the Royal Navy must be run up when the first line is ashore when coming alongside.

On 16 January 1899, commissioned ships of the United States Coast and Geodetic Survey were authorized to fly their own flag to distinguish them from ships of the United States Navy, with which they shared a common ensign. Although they continued to fly the same ensign as U.S. Navy ships, ships of the Survey flew the Coast and Geodetic Survey flag as a "distinguishing mark" until the newly created National Oceanic and Atmospheric Administration (NOAA) took over control of the Survey's ships in 1970. Since then, commissioned ships of NOAA, which also fly the same national ensign as U.S. Navy ships, have flown the flag of the National Oceanic and Atmospheric Administration as a distinguishing mark.

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