If the service core (which contains the elevator shafts) becomes too big, it can reduce the profitability of the building. Architects must therefore balance the value gained by adding height against the value lost to the expanding service core.[10] Many tall buildings use elevators in a non-standard configuration to reduce their footprint. Buildings such as the former World Trade Center Towers and Chicago's John Hancock Center use sky lobbies, where express elevators take passengers to upper floors which serve as the base for local elevators.