With the breakthrough of modernism, various efforts were undertaken to rationalize architecture and building processes using industrial principles. Few architects explored these as intensively as Walter Gropius, the founder of the Bauhaus. Before World War One, and increasingly in the interwar years, Gropius and a number of colleagues undertook various experiments that manifested in a series of projects, essays, model houses and Siedlungen. These were aimed at conceptually different goals, i.e., they followed two different categories of industrial logic: First, a flexible construction kit and, second, an assembly line serial production. This article traces the genesis of these two concepts and analyses their characteristics using these earl...