The jack of the United States is a maritime flag representing United States nationality flown on the jackstaff in the bow of its vessels. The U.S. Navy is a prime user of jacks, but they are also used by ships of the Coast Guard, Military Sealift Command, National Oceanic and Atmospheric Administration, and other governmental entities. "The jack is flown on the bow (front) of a ship and the ensign is flown on the stern (rear) of a ship when anchored or moored. Once underway, the ensign is flown from the main mast."[1] The primary jack design until September 11, 2002 was the blue canton with stars (the "union") from the U.S. national ensign. The blue, starred jack is referred to as the Union Jack, but should not be confused with the British Union Jack. Like the ensign, the number of stars on the jack has increased with each state admitted into the union. Rules for flying the jack are similar to the national ensign.

Since September 11, 2002, the Navy has instead flown the First Navy Jack, a flag bearing 13 red and white stripes, a rattlesnake and the motto "DONT TREAD ON ME", coming from the first jacks supposedly used by the Navy during the Revolutionary War. It is flown from the jackstaff from 08:00 to sunset while Navy ships are moored or at anchor. It is required to be the same size as the union of the ensign being flown from the stern of the ship. It is also flown from the yardarm during a general court-martial or court of inquiry.[2] During times when the ensign is at half mast, the jack is also at half mast. The jack is hoisted smartly and lowered ceremoniously in the same manner as the ensign, however the jack is not dipped when the ensign is dipped.[3]

Some exceptions to the use of the Union Jack have occurred in the case of the U.S. Navy. The most prominent being the use of the First Navy Jack by the U.S. Navy in honor of the country's Bicentennial and subsequently.[4] On June 3, 1999, the Secretary of the Navy authorized the flying of the Submarine Centennial Jack aboard US Navy submarines and sub tenders during the year 2000.[5]

No. of Stars	Design	Dates in General Use	Notes
0	or	January 8, 1776–June 14, 1777	There is little evidence this jack had the rattlesnake or motto as traditionally depicted (see First Navy Jack).
13		June 14, 1777–May 1, 1795	Examples of many layouts of the 13 star pattern exist. See US Flag for details.
15		May 1, 1795–July 3, 1818	War of 1812
20		July 4, 1818–July 3, 1819
21		July 4, 1819–July 3, 1820
23		July 4, 1820–July 3, 1822
24		July 4, 1822–July 3, 1836
25		July 4, 1836–July 3, 1837
26		July 4, 1837–July 3, 1845
27		July 4, 1845–July 3, 1846
28		July 4, 1846–July 3, 1847
29		July 4, 1847–July 3, 1848
30		July 4, 1848–July 3, 1851
31		July 4, 1851–July 3, 1858
32		July 4, 1858–July 3, 1859
33		July 4, 1859–July 3, 1861	Civil War
34		July 4, 1861–July 3, 1863
35		July 4, 1863–July 3, 1865
36		July 4, 1865–July 3, 1867
37		July 4, 1867–July 3, 1877
38		July 4, 1877–July 3, 1890
43		July 4, 1890–July 3, 1891
44		July 4, 1891–July 3, 1896
45		July 4, 1896–July 3, 1908	Sinking of the USS Maine Spanish-American War Great White Fleet
46		July 4, 1908–July 3, 1912
48		July 4, 1912–July 3, 1959	World War I and World War II
49		July 4, 1959–July 3, 1960
50		July 4, 1960—October 12, 1975[4]
		January 1, 1977—Sept 11, 2002[4][6]	From 1980, the oldest active ship in the navy flies the First Navy Jack instead[4]
		Sept 11, 2002 —	MSC and non-Navy vessels
0	First Navy Jack	October 13, 1975—December 31, 1976[4]	USN and US Bicentennials
		Sept 11, 2002[6]—	War on Terrorism USN vessels only (MSC and non-Navy vessels continue to fly the Union Jack) See First Navy Jack for explanation