The article looks at how cities in OECD countries are changing between 2001 and 2011. They found that urban populations are spreading out more, with growth happening outside city centers. Some cities are seeing more growth in their central areas, while others are developing multiple centers. Overall, more people are living in lower-density areas near the main city center, but not in it. Being close to smaller centers is also good for growth, showing that new central areas are forming in cities.