The Multiple nuclei model is a basic model in modern urban planning. It was by Chauncy Harris and Edward Ullman in 1945. It was the third model in the Chicago school (architecture). The main idea was that when a city reached a certain size its downtown central business district (CBD) could no longer serve the whole city. Shops and offices move to the outer parts of the city. They grow and attract other businesses. This causes the areas around them to grow into new business centers. 