Herbert and Katherine Jacobs Second House

The Herbert and Katherine Jacobs Second House (also known as Jacobs II or the Solar Hemicycle) is a house in Madison, Wisconsin, United States. Designed by Frank Lloyd Wright and built in 1946–1948, the house was designed for the journalist Herbert Jacobs and his wife Katherine, whose first house he had designed a decade earlier. The Solar Hemicycle name is derived from the house's semicircular-arc floor plan, its use of natural materials, and its energy-saving orientation. The Jacobs Second House has been praised for its architecture over the years, and Wright went on to design other houses with circular design motifs. The house was added to the National Register of Historic Places in 1974 and declared a National Historic Landmark in 2003.

The Jacobs family moved to western Madison, near Middleton, in 1942, and commissioned Wright to design them a new residence. After they rejected his first design, Wright proposed the Solar Hemicycle, which was partially embedded into an earthen berm. Although the Jacobses paid the first part of Wright's fee in early 1944, construction did not start for over two years. The Jacobses ended up constructing much of the house themselves because of contractor delays and a misunderstanding that prompted Wright to resign. The family moved into the house's root cellar in July 1948 and occupied the rest of the house that September.