The United States Capitol is the seat of Congress — the House of Representatives and the Senate — and the building whose dome defines Washington's skyline. The east-facing front has been the site of presidential inaugurations since Andrew Jackson's in 1829. The west-facing side looks down the National Mall toward the Lincoln Memorial and is the view that appears in most photographs.
Free tours of the Capitol interior are available through your Congressional representative's office or through the Capitol Visitor Center (below the east lawn). The tour includes the Rotunda, whose 180-foot dome is decorated with a fresco of George Washington ascending to the heavens as a god — a piece of official mythology that deserves to be seen. When Congress is in session the public galleries of both chambers are accessible. Security lines can be long; arrive early.