July 10, 1821: In accordance with the terms of the 1819 Adams–Onís Treaty, sovereignty over Spanish Florida is officially transferred to the United States from Spain.
The count below identifies party affiliations at the beginning of the first session of this congress. Changes resulting from subsequent replacements are shown below in the "Changes in membership" section.
Senate
During this congress, two Senate seats were added for the new state of Missouri.
For the beginning of this congress, six seats from Massachusetts were reapportioned to the new state of Maine (one seat had already moved during the previous congress), 3 Stat.555. During this congress, one House seat was added for the new state of Missouri, 3 Stat.547.
Senators were elected by the state legislatures every two years, with one-third beginning new six-year terms with each Congress. Preceding the names in the list below are Senate class numbers, which indicate the cycle of their election. In this Congress, Class 1 meant their term began with this Congress, requiring reelection in 1826; Class 2 meant their term ended with this Congress, requiring reelection in 1822; and Class 3 meant their term began in the last Congress, requiring reelection in 1824.
Selah Tuthill (DR) was elected after the Congress term had already begun, and died on September 7, 1821, before Congress met. It is uncertain whether credentials were ever issued for Tuthill.
^Selah Tuthill (DR) was elected in New York's 6th district late in April 1821 and died September 7, 1821 before Congress convened. It is unclear if/when he received his credentials.
^ ab When seated or oath administered, not necessarily when service began.
References
^Letter: Erskine College, [Due West, South Carolina], to John K. Gaillard, Double Branches, Anderson District, [South Carolina] by W. A. Thomson, 1849
