Building in Shaker was controlled by a set of restrictive covenants and building guidelines established by the Van Sweringens and known as Shaker Standards. Shaker Standards prevented the community from being developed in any way contrary to how the Van Sweringens intended, including barring African Americans. Standards limited commercial development, rental property development, and residence style and size. Standards set roof slope angles, materials, finishes, and garage placement. All residences were required to be unique and designed by an architect. Duplex residences in the community were restricted to designated areas and were required to follow guidelines designed to give the impression that a structure was a single-family home. By 1920, the Van Sweringens controlled more than 4,000 acres (16 km2) in the community, which reached city status in 1931. Since lots sold slowly, the brothers concluded that Shaker Heights needed a transportation system between the suburb and downtown.