ſ

An italicized long s used in the word "Congress" (as "Congreſs") in the United States Bill of Rights

The long s, ⟨ſ⟩, also known as the medial s or initial s, is an archaic form of the lowercase letter ⟨s⟩, found mostly in works from the late 8th to early 19th centuries. It replaced one or both of the letter s in a double-s sequence (e.g., "ſinfulneſs" for "sinfulness" and "poſſeſs" or "poſseſs" for "possess", but never "poſſeſſ"). The modern ⟨s⟩ letterform is known as the "short", "terminal", or "round" s.