Oak Park: The Grammar of the Prairie House

Frank Lloyd Wright's two decades in Oak Park did not produce a style of houses. They produced a grammar. The first move in that grammar is the easiest one to underread. A Prairie house looks long. The horizontal mass is the eye's first take on it, and on most visitors it stops there. It should not. The horizontality is a structural argument about how a house should sit on the ground in the Midwest, what its roof should do, where its hearth should be, and what its windows should look at. Walk the residential blocks of Oak Park between Forest Avenue and East Avenue, between Lake Street and Chicago Avenue, and you can watch the argument get assembled in three dimensions, one house at a time, between 1889 and 1909.

The architect was Frank Lloyd Wright. He was twenty-two when he bought the corner lot at Forest and Chicago Avenues in 1889 with a five-thousand-dollar loan from his employer Louis Sullivan, and he was forty-one when he left Oak Park for Europe in 1909. In those twenty years he built and rebuilt his own home and studio, and he designed roughly two dozen Prairie-style houses within walking distance of that corner. The Frank Lloyd Wright Trust runs his home and studio as a museum now. The other houses are private. Most of them are still occupied. All of them are visible from the sidewalk.

What he assembled in Oak Park, working alone for the first half of the period and then with a small studio cohort (including Marion Mahony, Walter Burley Griffin, Barry Byrne, William Drummond, all of whom would later carry the work outward), is what the architectural literature calls the Prairie style. The term is loose. The grammar underneath it is exact. Five moves, repeated and varied across two dozen houses, all visible in the surviving cluster.

The first move: horizontality as a structural claim

A typical American Victorian house of the 1880s was vertical. The walls ran up at least two stories, the gables made the roof a sharp peak, the windows were tall narrow pairs, the ornament emphasized the rise. The Prairie house reversed every one of these decisions on purpose.

The walls stop running up. Two stories at most, often a story and a half, with the upper story half-buried under a low hipped roof. The roof, when hipped, sits with broad eaves that overhang the wall by several feet. The eaves cast the wall into shadow at noon. The shadow line becomes part of the design. The windows run in continuous horizontal bands rather than vertical pairs. The ornamental belt courses of brick or wood emphasize the horizontal. Stand on Forest Avenue in front of the Arthur Heurtley House (1902, 318 North Forest) and the entire reading lives in the eaves. The roof is so deep that the upper story barely shows. The brick belt course at the base of the second story extends the horizontal line. The shadow from the eave runs across the front like a second roof.

The structural claim under the move is that the house should match the land. Wright's polemical version of the argument, written later, is that the prairie is horizontal and a house built on it should reflect the geometry of the ground. The carpentry under the claim is that a deep eave on a wood-framed house requires cantilevered framing: the roof joists extend past the wall by four or six or eight feet, anchored back inside by counterweight at the ridge. Cantilever is the verb. The eave is the noun. Once you know to look for it, you can read a Prairie house from the eave alone.

The second move: the ribbon of art glass

The windows are not single sheets. They are runs, often six or eight or twelve windows in a horizontal row, each one small, each one set with leaded clear-and-colored glass in a geometric pattern. The Frank W. Thomas House at 210 North Forest (1901) shows the assembly in its purest form: bands of small geometric windows wrap the second story, broken only by structural piers. The glass is the second sheath. The first sheath, the eaves, is for shadow. The second sheath, the windows, is for light filtering inside.

Wright designed almost all the glass himself. The patterns are abstract, mostly orthogonal, derived from geometric vocabularies of late-nineteenth-century European decorative arts but stripped of the European narrative imagery. They read as both ornament and ventilation. A Prairie house is windowless in the conventional sense (no large picture window punched into a wall) and windowed in another sense (long bands of small glazed openings, often operable, often running the length of the room behind them). Air moves. Light moves. The eye outside reads a horizontal band rather than individual openings.

The third move: the hearth as plan-center

Walk inside any preserved Prairie interior. The Wright Home and Studio, on guided tour, is the cleanest example. The fireplace is not a feature against a wall. The fireplace is the center of the plan. The other rooms organize around it. The living room flows into the dining room. The dining room flows into a small study. There are no clear doorways. The walls between the rooms stop short of the ceiling or stop short of each other. The plan opens.

This was a complete break with the American Victorian floor plan, which was a sequence of small closed rooms strung along a central hall. Wright's argument, which he made repeatedly in print, was that the hearth should be the symbolic and physical center of the home and that the rooms should radiate from it rather than line up like train cars. The argument carried a structural cost. An open plan requires longer floor beams (because there are fewer walls to break the span) and a stiffer roof structure (because there is less shear bracing from interior walls). Wright designed the structure to take the cost. The brick chimney mass, set in the middle of the house, became the structural pier as well as the symbolic center.

You can read the move from outside. A Prairie house's chimney is almost always at the center of the roof, not at an end. The chimney mass is the spine. The roof hipping on either side mirrors around it.

The fourth move: integrated ornament

Wright did not believe in ornament applied after construction. He believed in ornament built into the structure. The Roman brick walls of the Edwin H. Cheney House at 520 North East Avenue (1903) are the clearest example. The brick is unusually long and shallow, with mortar joints raked deeper on the horizontal courses than on the vertical, so the eye reads the horizontal lines and softens the verticals. The brick itself is the ornament. There is no decoration applied. The pattern of the mortar joint, the choice of the brick coursing, the depth of the rake: every detail extends the horizontal claim that the roof makes overhead.

The same principle runs through the art glass, the integrated wood trim inside, the built-in furniture (chairs, dining tables, light fixtures, often designed for the specific house and built into its walls), and even the landscape: foundation plantings stay low, the lawn runs to the sidewalk, no hedge separates the house from the street. The house claims the lot and the lot claims the house.

The fifth move: the public building

The four moves work for a single-family house. The question was what would happen if you scaled them up. The answer is Unity Temple at 875 Lake Street, finished in 1908. The Unitarian congregation's wood-frame church had burned in 1905. The congregation, with little money, asked Wright to rebuild. He proposed a building of poured reinforced concrete with the concrete left as the finished surface. It would be the first major reinforced-concrete public building in America with exposed concrete as the surface. Wright considered it his first modern building. Most architectural historians have agreed.

Stand at the corner of Lake and Kenilworth and look at the building. The mass is a cubic block, with the cube of the worship space facing Lake and the cube of the social hall facing east, connected by a low entry block. The horizontal eaves of the Prairie house are here, scaled up to civic proportions. The art glass of the Prairie windows is here, set into clerestory panels in the worship space's ceiling, washing the interior with colored light from above. The hearth-as-center of the Prairie plan is here, restated as a central worship cube with seating banked around it on three sides, no axial aisle, no front-of-room hierarchy. The integrated ornament is here, in the cast-concrete piers and the wood trim of the interior.

The grammar holds. A church can be designed by the same rules as a house. Unity Temple is the proof.

UNESCO inscribed Unity Temple as a World Heritage Site in 2019, alongside seven other Wright buildings from across his career. The designation is the international architectural community's formal recognition of what Wright assembled in Oak Park.

What the cluster teaches

The literature sometimes treats Wright as a solitary genius working through individual buildings. The Oak Park cluster argues the opposite. The houses are variations on a grammar that Wright was refining in public, in front of his neighbors, in a single suburban village over twenty years. Some experiments worked. The Heurtley House is the canonical Prairie specimen, the one his colleagues and clients pointed to when they wanted to show the style. Some experiments were awkward. The Moore-Dugal House at 333 North Forest, built for a conservative client in 1895 in a Tudor Revival idiom Wright was openly unenthusiastic about, then partially rebuilt by Wright after a 1922 fire, holds two grammars stacked uneasily on the same lot.

What the cluster teaches, walked one block after another, is that style is not the right word for what Wright was doing. Style implies a set of decorative preferences. Grammar implies a set of rules that produce sentences. The Prairie houses are sentences. They are not all good sentences. They are all written with the same alphabet. After walking the cluster, that alphabet is portable. Drive through any inner-ring American suburb built between 1900 and 1940 and the houses that read as Prairie (low hipped roof, deep eaves, horizontal window bands, central chimney, integrated ornament) are using the alphabet that was assembled here.

The grammar travels. The cluster is where it was written down.