Area codes are added by two principal methods, number plan area "splits" and "overlay plans". Splits were implemented by dividing an area into two or more regions, one of which retained the existing area code and the other areas receiving a new code. In an overlay, multiple codes are assigned to the same geographical area, obviating the need for renumbering of existing services. Subtle variations of these techniques have been used as well, such as "dedicated overlays", in which the new code is reserved for a particular type of service, such as cellular phones and pagers, and "concentrated overlays", in which a part of the area retained a single code while the rest of the region received an overlay code. The only service-specific overlay in the NANP was area code 917 (New York City) when it was first installed; such service-specific area code assignments were later prohibited by the Federal Communications Commission.