The Treasury building west wing, facing the White House, was officially completed in 1864 during the Civil War.
The Treasury building and the expansion of the U.S. Capitol building and it's dome were among the few large building projects to remain active during the Civil War.
Although the majority of the west wing was completed and able to be occupied in 1864, a few areas such as the stairwell in the northwest corner, were not finished until 1869.
Unable to be seen from the outside, the west wing has a double spiral staircase leading up to a large, oval three-part dome with skylights, the largest such dome dating from the 19th century still in place in the United States.
The west wing of the Treasury building faces the White House, for many years separated by a public street leading south to the Washington Monument. Due to limited access, the west wing is now the least visible of the four wings of the building.