Abuja is the capital and eighth most populous city of Nigeria. Situated at the centre of the country within the Federal Capital Territory, it is a planned city built mainly in the 1980s based on a master plan by International Planning Associates, a consortium of three American planning and architecture firms made up of Wallace, Roberts, McHarg & Todd as the lead, Archisystems International, and Planning Research Corporation. The Central Business District of Abuja was designed by Japanese architect Kenzo Tange. It replaced Lagos, the country's most populous city, as the capital on 12 December 1991.