The density gradient of industrialising cities has tended to follow a specific pattern: the density of the centre of the city would rise during urbanisation and the population would remain heavily concentrated in the city centre with a rapid decline in settlement towards the periphery. Then, with continued economic growth and the expanding networks of public transport, people (particularly the middle-class) would then slowly migrate towards the suburbs, gradually softening the population density gradient. This point was generally reached when the city reached a certain stage of economic development. In London, this point was reached in the first half of the nineteenth century, in Paris toward the end of the century and in New York City at the turn of the twentieth.