Richardsonian Romanesque is a style of Romanesque Revival architecture named after architect Henry Hobson Richardson (1838–1886), whose masterpiece is Trinity Church, Boston (1872–1877), designated a National Historic Landmark. Richardson first used elements of the style in his Buffalo State Asylum for the Insane in Buffalo, New York, designed in 1870.

History and development
This very free revival style incorporates 11th and 12th century southern French, Spanish and Italian Romanesque characteristics. It emphasizes clear, strong picturesque massing, round-headed “Romanesque” arches, often springing from clusters of short squat columns, recessed entrances, richly varied rustication, blank stretches of walling contrasting with bands of windows, and cylindrical towers with conical caps embedded in the walling.

Architects working in the style
The style includes work by the generation of architects practicing in the 1880s before the influence of the Beaux-Arts styles. It is epitomised by the American Museum of Natural History’s original 77th Street building by J. Cleaveland Cady of Cady, Berg and See in New York City. It was seen in smaller communities in this time period such as in St. Thomas, Ontario’s city hall and Menomonie, Wisconsin’s Mabel Tainter Memorial Building, 1890.

Some of the practitioners who most faithfully followed Richardson’s proportion, massing and detailing had worked in his office. These include:

Alexander Wadsworth Longfellow and Frank Alden (Longfellow, Alden & Harlow of Boston & Pittsburgh);
George Shepley and Charles Coolidge (Richardson’s former employees, and his successor firm, Shepley, Rutan & Coolidge of Boston);
Herbert Burdett (Marling & Burdett of Buffalo).
Other architects who employed Richardson Romanesque elements in their designs include:

Spier and Rohns and George D. Mason, both firms from Detroit;
Edward J. Lennox and John Wellborn Root, Toronto-based architects who derived many of their designs from the Richardson Style;
Harvey Ellis designed in this style in Minneapolis, Minnesota.
Fenimore C. Bate designed the Grays Armory in this style in Cleveland, Ohio.
The style also influenced the Chicago school of architecture and architects Louis Sullivan and Frank Lloyd Wright. In Finland, Eliel Saarinen was influenced by Richardson.

Dispersion
Research is underway to try to document the westward movement of the artisans and craftsmen, many of whom were immigrant Italians and Irish, who built in the Richardsonian Romanesque tradition. The style began in the East, in and around Boston, where Richardson built the influential Trinity Church on Copley Square. As the style was losing favor in the East, it was gaining popularity further west. Stone carvers and masons trained in the Richardsonian manner appear to have taken the style west, until it died out in the early years of the 20th century.

As an example, four small bank buildings were built in Richardsonian Romanesque style in Osage County, Oklahoma, during 1904–1911.

Source From Wikipedia