This flag flew from 4 July 1818 to 3 July, 1819. States 16 through 20 admitted to the Union were in sequential order, 16, 17, 18, 19, and 20.
There was actually a bit of a lull between the 15 and 20 star flags. The US government couldn't exactly decide what to do with the flag each time a new state was added. It seemed impractical to add stars and stripes to the flag every time a new state was admitted to the Union. So, between 1796 when Tennessee was admitted, and 1817 when Mississippi joined, nothing was done. Finally in 1818, it was decided to go back to the original 13 stripe pattern and simply add stars to the canton each time a new state joined the Union.
Had this decision been made in 1796, there would have been a 16 star flag, and then later a 17, 18, and 19 star flag, and each of them with the exception of the 19 star flag would have flown more than a year. The 16 star flag would have flown 8 years, from 1796 to 1803. The 17 star flag would have flown 9 years from 1803 to 1812. The 18 star flag would have flown 5 years from 1812 to 1817, while the 19 star flag would have had a life span of 1 year, flying from 1817 to 1818.
, 23 February 2007
A 20 star 13 stripe flag with the stars in the "Great Star" pattern was raised in Washington D.C. on or shortly after 13 April 1818, less than two weeks after the adoption of the new flag law, which was adopted on 4 April 1818. However, in a strictly legal sense, the flag, under the terms of the law, was not official until 4 July 1818. Then, again, the prior flag flown there, which had 18 stars and 18 stripes, was not "official" under the 1795 flag law, even though used in an "official" capacity.
The actual text of the Flag Act of 1818 is:
Sect. 1. Be it enacted by the Senate and House of Representatives of the United States of America in Congress assembled, That from and after the fourth day of July next, the flag of the United States be thirteen horizontal stripes, alternate red and white; that the union have twenty stars, white in a blue field.
Sect. 2. And be it further enacted, That on the admission of every State into the Union, one star be added to the union of the flag; and that such addition shall take effect on the fourth day of July next succeeding such admission.
Approved, April 4, 1818.
, 21 August 2005