The central business district (CBD) is the area within a city that functions as the center of business and finance. It is characterized by an above average spatial concentration of tall office buildings with corporate headquarters and financial institutions. CBDs also include important cultural and political institutions in addition to businesses with a more recreational function, such as hotels and restaurants. Most of the employees commute into these high priced areas, which are characterized by an employment structure comprising mobile employees in low paid, precarious jobs in the service sector, like cleaning or wait staff and high paid jobs for professionals. Some CBDs form the historical core of the city. They are represented as the city's powerhouse and are visually present in the skyline. Other cities have more than one CBD and, furthermore, not all CBDs are located in the historical center of their city. They are globally interconnected by flows of communication, finance, goods, or migration.