Abstract The spatial ordering of Rio de Janeiro’s poor suburbs, through subdivision (loteamento), did not raise such interest among anthropologists. Based on documents, mostly urban plans, and legislation, we intend to make a “genealogy” (Foucault) of the subdivision, giving attention to technical and political rationalities. We argue that the subdivision owes much to the “graphic reason” (Goody), to the kind of operations that the use of “graphic artifacts” allows. Moreover, this technology responds to specific problems: during colonial times, government regulated street opening in order to defend public spaces. At the end of the eighteenth century, the problem was how to reform the road network. Current subdivision regulations come from the early twentieth century, when new issues about collective needs aroused. Far from being an illegal city, poor suburbs appear as a laboratory for urban planning.