Despite all good intensions, a history of apartments has never been written. History books may be full of specimens of evolving forms of flatted living, but they never tell the story of their speciation, that is, the process in which apartments came about as a specific form of residential architecture. This lacking formational history was first pointed out by nineteenth century architect Sydney Perks in his progressive design manual for Residential Flats. Yet even a hundred years later, American social historians who investigated how apartment living had transformed urban life, noted that although apartments had been the subject of ample scholarly studies, the history of apartments remains “a story in need of telling”. In her study Alone Together, Elizabeth Cromley stresses that the real virtues of apartments are namely “not ones that architectural historians have usually identified as significant.” As she argues, contrary to the art-historical concern with stylistic, or periodic distinctions, the history of apartments calls for a more affirmative attention to the socially emancipating relationships that architecture historically enabled; a social history of the differences apartments made for women. But beyond being simply a female or feminist concern, this approach implies to also understand more generally the differences apartments made within our forms of living together and relating to another. This essay briefly maps a possible starting point to advance the historical formation of apartments in such a relational approach to architecture, arguing that it hinges on a fine difference in the very conception of our relation of living together/apart.