Many cities are served by more than one airport, typically to avoid congestion, and where there may be factors preventing expansion of existing airports. In other cities, multiple airports may be built to cater for different uses, such as international and domestic flights. (For instance domestic flights may use smaller aircraft which can use a shorter runway.) The following lists cities which are served by more than one airport offering scheduled passenger services. Airports are included even if they are not within the city boundaries. Military airbases (without passenger service) and airports with only charter flights or cargo service are not included.