Answer: At Congress Hall in Philadelphia.
From 1790 to 1800, Philadelphia served as the temporary capital of the United States and during this period the federal Congress convened at Sixth and Chestnut Streets. First called the Philadelphia County Building, it was renamed Congress Hall in 1790 after the first congressional meeting there. The plans for the building were set forth by Andrew Hamilton in the 1730s; but construction did not commence until 1787 and the building was completed two years later.
While meeting in New York City, Congress passed a bill that designated land on the Potomac River for the permanent capital, later to be known as the City of Washington, in the District of Columbia. The transfer of the government to the new capital was fixed for the year 1800. In the interim, and due in part to the efforts of Pennsylvania Senator Robert Morris, famously called the “financier of the Revolution,” Philadelphia was named the temporary capital starting on December 6, 1790. The House of Representatives met on the first floor of the building and the Senate on the upper floor.
In the next decade, many of the basic tenets of government under the new Constitution were set forth in Philadelphia. Congress Hall hosted the inaugurations of two presidents: George Washington in 1793 (his second term) and John Adams in 1798. Vermont, Kentucky, and Tennessee were admitted to the Union in Congress Hall. Finally, the Bill of Rights was ratified while Congress met in these two rooms.
Today Congress Hall is one of the five buildings of the Independence Hall Group, part of the Independence National Historical Park.
More images of Congress Hall can be found in HSP’s Society print collection (V89). HSP’s library also contains published works on the buildings, such as A Short History of Congress Hall (call number UPA/Pam F 158.8.I3 I532, no. 4).