Article 1, Section 5, Clause 4: Adjournment

Neither House, during the Session of Congress, shall, without the Consent of the other, adjourn for more than three days, nor to any other Place than that in which the two Houses shall be sitting.

This clause establishes that the two Houses are to meet concurrently in time and space. That is, a session of Congress is a session of both houses of Congress, and it is strongly implied that they are normally to meet at the same location. This location is presently the Capitol in Washington, which has two wings, one for each House. The Houses meet on the second floor, with the South Wing holding the Hall of the House of Representatives and the North Wing holding the Senate Chamber.

It is not uncommon for one or both Houses to hold pro forma sessions every three days in which no business is conducted, but the requirement to be considered still in session is met. This has been done to prevent the President from pocket vetoing bills, or making recess appointments, or exercising his power to call Congress into special session.