What year marked the beginning of significant changes to the Capitol structure?

The year 1851 marks the beginning of significant changes to the Capitol structure primarily due to the extensive renovations and expansions that were undertaken at that time. After the Capitol suffered multiple threats, including fires, there was a pressing need to restore and enhance the building.

In 1851, the construction of the new dome began, which later became one of the most recognizable features of the Capitol. This monumental project not only changed the exterior appearance of the building but also symbolized a new era in the architectural development of the Capitol. The completion of the dome in the following years solidified its status as an iconic representation of American democracy.

Other years listed, such as 1840 and 1865, were significant for various reasons but did not initiate the major structural changes that occurred starting in 1851. 1875 does not represent a key turning point in the architectural evolution of the Capitol either, as the more impactful changes were well underway by then.