Studies of working people have long been framed by the concepts of “free” and “unfree” labour, a pair that distinguishes workers who are fully proletarianized from those who are not. Proletarians are working people without property, and therefore compelled to sell their capacities for money, but at the same time personally free to choose whom to sell their capacities to. Such (double-) free labour is contrasted with labour that is unfree, either because workers are compelled to offer their capacities to specific takers – under conditions established by those takers rather than by means of a labour market – or because these workers are not completely propertyless. Throughout the twentieth century, a dominant assumption among labour historians has been that the two concepts of free and unfree labour also reflect a basic trend in modern world history: the progressive replacement of unfree labour by free labour – or the progressive proletarianization of the world's workers.