Neighbourhoods in preindustrial cities often had some degree of social specialisation or differentiation. Ethnic neighbourhoods were important in many past cities and remain common in cities today. Economic specialists, including craft producers, merchants, and others, could be concentrated in neighbourhoods, and in societies with religious pluralism neighbourhoods were often specialised by religion. One factor contributing to neighbourhood distinctiveness and social cohesion in past cities was the role of rural to urban migration. This was a continual process in preindustrial cities, and migrants tended to move in with relatives and acquaintances from their rural past.