Decorum refers to the suitability of a building's design and was a commonplace principle of architectural theory from the Renaissance to the beginnings of modernism. It was relevant to ornament, shaping the way a building articulated its status within civic and social order. This essay examines decorum as part of the history of ideas, with phases of growth, codification, and decline. Its fading was not unresisted, being part of a critical debate that emerged in the wake of the Industrial Revolution – namely, the role architecture might play in creating a cohesive environment for the modern world.