The United States Flag Code, a federal law, states that an American flag displayed vertically against a wall or in a window should have the union to the observer's left. The union refers to the cluster of white stars in the blue block at the top.
When hanging the flag vertically over a street, the union should be to the east on a north-south street or to the north on an east-west street. A flag hanging vertically at a right angle to a building should have the union away from the wall. A flag displayed vertically in the lobby or corridor of a building with a single main entrance should have the union to the left. If a building has more than one main entrance, the flag should be displayed in the center of the lobby, with the union to the north if the entrances are east and west and to the east if the entrances are north and south. The union should be to the east if there are entrances on more than two sides. If a flag is displayed flat on a dais, it should be above and behind the speakers. In all cases, the union is always on the top; a flag flown upside-down denotes an emergency.