Though it is associated with classic American architecture from Washington, D.C. to Boston, Mass., the Federal style of architecture was actually created in Scotland by the Adams Brothers, who based it on houses and plans in Italy. The style became known as Federal because it gained popularity in the early days of the United States.
Identifying features of Federal style architecture include a fanlight over the door (almost always rounded, rarely squared); Classical/Greek detailing in entryways; Palladian windows; and balustrades. As in Georgian architecture, symmetry and double-hung sash windows are key elements. Some high-style examples of Federal style feature oval and circular rooms – the most famous example of this is the Oval Office.
Image (Evermay, Georgetown) courtesy CurbedDC. Info courtesy Dr. Tom Paradis, Northern Arizona University.