Naval jack

A jack is a national (originally naval) flag flown from a short jackstaff at the bow of a vessel, while the ensign is flown on the stern. Jacks on bowsprits or foremasts appeared in the 17th century. The word "jack" is said to result from the signature Jacques of King James I in whose reign (1603–1625) the Union Jack was designed. A country may have different jacks for different purposes, especially when (as in the United Kingdom and the Netherlands) the naval jack is forbidden to other vessels. The United Kingdom has an official civil jack; the Netherlands has several unofficial ones. In some countries, ships of other government institutions may fly the naval jack, e.g. the ships of the U.S. Coast Guard and the National Oceanic and Atmospheric Administration in the case of the jack of the United States. Certain organs of the British government have their own departmental jacks. Commercial or pleasure craft may fly the flag of an administrative division (state, province, land) or municipality at the bow. Merchant ships may fly a house flag. Yachts may fly a club burgee or officer's flag or the owner's private signal at the bow. Practice may be regulated by law, custom, or personal judgment.

A naval jack is usually flown when the ship is not under way, but is moored or at anchor, or when it is dressed overall on special occasions. The Union Jack of the Royal Navy must be run up when the first line is ashore when coming alongside. The same regulations are applied by the Royal Canadian Navy

In the United States, the First Navy Jack is also used by the United States Naval Special Warfare Command as uniform patches.

...
Wikipedia